diff --git a/src/lib/server/delivery-notes-service.test.ts b/src/lib/server/delivery-notes-service.test.ts
index db97218..6af3676 100644
--- a/src/lib/server/delivery-notes-service.test.ts
+++ b/src/lib/server/delivery-notes-service.test.ts
@@ -705,4 +705,75 @@ describe("setDeliveryNoteOrder", () => {
     expect(String(stored?.notes ?? "")).toMatch(/existing dn note/);
     expect(String(stored?.notes ?? "")).toMatch(/Hermes linked this delivery note to order order-2/);
   });
+
+  // Regression coverage for the delivery-notes backfill (2026-09-07):
+  // DNs created while the /api/invoicing/delivery-note `fields`+`expand`
+  // bug was live (see route.ts) got saved with customer/company/order
+  // all blank. Re-linking the order via this setter should also
+  // backfill customer/company from that order, since they're supposed
+  // to mirror it — see the docstring above `setDeliveryNoteOrder`.
+  it("backfills customer/company from the order when the DN's own values are blank", async () => {
+    const dnId = seedDeliveryNote({ status: "draft", order: "", customer: "", company: "" });
+    getOrCreate("orders").set("order-2", {
+      id: "order-2",
+      order_number: "ORD-2",
+      customer: "customer-9",
+      company: "company-9",
+    });
+
+    const result = await setDeliveryNoteOrder({
+      deliveryNoteId: dnId,
+      orderId: "order-2",
+    });
+
+    expect(result.deliveryNote.customer).toBe("customer-9");
+    expect(result.deliveryNote.company).toBe("company-9");
+    const stored = getOrCreate("delivery_notes").get(dnId);
+    expect(stored?.customer).toBe("customer-9");
+    expect(stored?.company).toBe("company-9");
+  });
+
+  it("backfills customer/company even when the order is already linked (idempotent-on-order path)", async () => {
+    // A DN can already point at the right order (e.g. re-running this
+    // setter a second time) yet still be missing customer/company —
+    // exactly the shape the fields+expand regression left behind.
+    // The idempotent-on-order branch must still apply the backfill.
+    const dnId = seedDeliveryNote({ status: "draft", order: "order-2", customer: "", company: "" });
+    getOrCreate("orders").set("order-2", {
+      id: "order-2",
+      order_number: "ORD-2",
+      customer: "customer-9",
+      company: "company-9",
+    });
+
+    const result = await setDeliveryNoteOrder({
+      deliveryNoteId: dnId,
+      orderId: "order-2",
+    });
+
+    expect(result.previous).toEqual({ order: "order-2" });
+    expect(result.deliveryNote.customer).toBe("customer-9");
+    expect(result.deliveryNote.company).toBe("company-9");
+  });
+
+  it("does not overwrite a customer/company the DN already has", async () => {
+    const dnId = seedDeliveryNote({
+      status: "draft",
+      order: "",
+      customer: "customer-1",
+      company: "company-1",
+    });
+    getOrCreate("orders").set("order-2", {
+      id: "order-2",
+      order_number: "ORD-2",
+      customer: "customer-9",
+      company: "company-9",
+    });
+
+    await setDeliveryNoteOrder({ deliveryNoteId: dnId, orderId: "order-2" });
+
+    const stored = getOrCreate("delivery_notes").get(dnId);
+    expect(stored?.customer).toBe("customer-1");
+    expect(stored?.company).toBe("company-1");
+  });
 });
diff --git a/src/lib/server/delivery-notes-service.ts b/src/lib/server/delivery-notes-service.ts
index d243de1..ab0942c 100644
--- a/src/lib/server/delivery-notes-service.ts
+++ b/src/lib/server/delivery-notes-service.ts
@@ -512,6 +512,19 @@ export interface SetDeliveryNoteOrderInput {
  * WFDASH UI surfaces which changes came from the agent. The
  * `notes` field on `delivery_notes` is free-text (not a JSON
  * feed like `order_items.notes`).
+ *
+ * Customer/company backfill: `delivery_notes.customer` /
+ * `.company` are meant to mirror the linked order's customer /
+ * company (see the header-create block in the legacy
+ * `/api/invoicing/delivery-note` route). A DN created while those
+ * relations failed to resolve (e.g. the 2026-08-30 → 2026-09-07
+ * `fields`-strips-`expand` regression — see git history on that
+ * route) ends up with `customer`/`company`/`order` all blank. When
+ * this setter links the order, it also fills `customer`/`company`
+ * from that order IF the DN's own values are currently blank —
+ * never overwrites an operator-set value. This makes re-linking
+ * the order via this one call enough to fully backfill an affected
+ * DN, instead of needing a second, nonexistent setter.
  */
 export async function setDeliveryNoteOrder(
   input: SetDeliveryNoteOrderInput,
@@ -543,9 +556,14 @@ export async function setDeliveryNoteOrder(
   }
 
   // 404 if the order doesn't exist. PB would 400 on a bad FK id,
-  // but a 404 from getOne is a cleaner signal for the caller.
+  // but a 404 from getOne is a cleaner signal for the caller. The
+  // record is kept (not discarded) — its `customer`/`company`
+  // relation ids feed the backfill below.
+  let order: { customer?: string; company?: string } | null = null;
   try {
-    await pb.collection(ORDERS).getOne(orderId);
+    order = await pb
+      .collection(ORDERS)
+      .getOne<{ customer?: string; company?: string }>(orderId);
   } catch {
     const err = new Error(`Order ${orderId} not found`);
     (err as Error & { status?: number }).status = 404;
@@ -574,9 +592,29 @@ export async function setDeliveryNoteOrder(
 
   const previous: { order: string } = { order: dn.order || "" };
 
-  // Idempotent no-op when the DN already points at this order.
-  // Don't write, don't stamp — just return the DN as-is.
+  // Backfill customer/company from the order — only when the DN's
+  // own value is currently blank. See the docstring above. Computed
+  // up front so it also applies on the idempotent (order-unchanged)
+  // path below: a DN can already point at the right order yet still
+  // be missing customer/company (exactly the shape the 2026-08-30 →
+  // 2026-09-07 `fields`-strips-`expand` bug left behind), and a
+  // second call with the same order id is the natural way an
+  // operator/agent re-runs this setter to fix that up.
+  const backfill: Record<string, unknown> = {};
+  if (!dn.customer && order?.customer) {
+    backfill.customer = order.customer;
+  }
+  if (!dn.company && order?.company) {
+    backfill.company = order.company;
+  }
+
+  // Idempotent on `order`: don't re-write it, don't re-stamp notes —
+  // but still apply the customer/company backfill if there's one to
+  // apply, since that's a pure data-quality fix, not a "change".
   if (previous.order === orderId) {
+    if (Object.keys(backfill).length > 0) {
+      await pb.collection(DELIVERY_NOTES).update<DeliveryNote>(deliveryNoteId, backfill);
+    }
     const fresh = await pb
       .collection(DELIVERY_NOTES)
       .getOne<DeliveryNote>(deliveryNoteId);
@@ -586,7 +624,7 @@ export async function setDeliveryNoteOrder(
   // Optional actor stamp: append a single-line audit entry onto
   // the DN's free-text notes field. We always also update the
   // `order` relation; the stamp is just an audit breadcrumb.
-  const update: Record<string, unknown> = { order: orderId };
+  const update: Record<string, unknown> = { order: orderId, ...backfill };
   if (input.actor?.id && input.actor?.name) {
     const stamp = `[${new Date().toISOString()}] ${input.actor.name} linked this delivery note to order ${orderId}`;
     const existingNotes = (dn.notes ?? "").toString();