diff --git a/src/app/api/insights/layout/route.ts b/src/app/api/insights/layout/route.ts
index cc59124f1..88a242ece 100644
--- a/src/app/api/insights/layout/route.ts
+++ b/src/app/api/insights/layout/route.ts
@@ -170,7 +170,12 @@ export const PUT = apiHandler(async (request: NextRequest) => {
     return returnAllZodIssues(parsed.error, 422);
   }
 
-  const normalized = serializeInsightsLayout(parsed.data);
+  // v1.16.13 — load the user's stored layout so a PUT that omits a
+  // dimension (iOS reorders tiles with a tiles-only body, no `sections`
+  // key) preserves the other dimension's customization instead of
+  // resetting it to defaults.
+  const previous = await buildInsightsLayout(user.id);
+  const normalized = serializeInsightsLayout(parsed.data, previous);
 
   await prisma.user.update({
     where: { id: user.id },
diff --git a/src/lib/__tests__/insights-layout.test.ts b/src/lib/__tests__/insights-layout.test.ts
index 8813c96e3..96c67f823 100644
--- a/src/lib/__tests__/insights-layout.test.ts
+++ b/src/lib/__tests__/insights-layout.test.ts
@@ -209,6 +209,86 @@ describe("serializeInsightsLayout — defaults + dense order", () => {
   });
 });
 
+describe("serializeInsightsLayout — stored-dimension preservation (v1.16.13)", () => {
+  it("keeps the stored sections when a PUT omits sections (tiles-only)", () => {
+    // The user previously customised sections (a hidden + reordered set).
+    const stored = serializeInsightsLayout({
+      version: 2,
+      sections: [
+        { id: "trends", visible: true, order: 0 },
+        { id: "vitals", visible: false, order: 1 },
+      ],
+      tiles: [{ id: "overview", visible: true, order: 0 }],
+    });
+
+    // An iOS tiles-only reorder PUT — no `sections` key.
+    const serialized = serializeInsightsLayout(
+      {
+        version: 1,
+        tiles: [
+          { id: "weight", visible: true, order: 0 },
+          { id: "overview", visible: true, order: 1 },
+        ],
+      },
+      stored,
+    );
+
+    // Stored sections survive verbatim instead of resetting to defaults.
+    expect(
+      serialized.sections.map((s) => ({
+        id: s.id,
+        visible: s.visible,
+        order: s.order,
+      })),
+    ).toEqual([
+      { id: "trends", visible: true, order: 0 },
+      { id: "vitals", visible: false, order: 1 },
+    ]);
+    // The PUT's tiles still apply.
+    expect(serialized.tiles.map((t) => t.id)).toEqual(["weight", "overview"]);
+  });
+
+  it("keeps the stored tiles when a PUT omits tiles (sections-only)", () => {
+    const stored = serializeInsightsLayout({
+      version: 2,
+      sections: [{ id: "vitals", visible: true, order: 0 }],
+      tiles: [
+        { id: "weight", visible: true, order: 0 },
+        { id: "bloodPressure", visible: false, order: 1 },
+      ],
+    });
+
+    const serialized = serializeInsightsLayout(
+      {
+        version: 2,
+        sections: [
+          { id: "trends", visible: true, order: 0 },
+          { id: "vitals", visible: false, order: 1 },
+        ],
+      },
+      stored,
+    );
+
+    // Stored tiles survive verbatim instead of resetting to defaults.
+    expect(
+      serialized.tiles.map((t) => ({ id: t.id, visible: t.visible })),
+    ).toEqual([
+      { id: "weight", visible: true },
+      { id: "bloodPressure", visible: false },
+    ]);
+    expect(serialized.sections.map((s) => s.id)).toEqual(["trends", "vitals"]);
+  });
+
+  it("falls back to defaults for an absent dimension when no stored layout is supplied", () => {
+    const serialized = serializeInsightsLayout({
+      version: 2,
+      tiles: [{ id: "overview", visible: true, order: 0 }],
+    });
+    // First-ever tiles-only PUT still persists a complete v2 blob.
+    expect(serialized.sections.length).toBe(INSIGHTS_SECTION_IDS.length);
+  });
+});
+
 describe("orderedVisibleSectionIds — W2 render-order helper", () => {
   it("returns every section in default order for the default layout", () => {
     expect(orderedVisibleSectionIds(DEFAULT_INSIGHTS_LAYOUT)).toEqual([
diff --git a/src/lib/insights-layout.ts b/src/lib/insights-layout.ts
index 501136496..412af7f9b 100644
--- a/src/lib/insights-layout.ts
+++ b/src/lib/insights-layout.ts
@@ -300,7 +300,15 @@ export interface InsightsLayoutInput {
 
 export function serializeInsightsLayout(
   layout: InsightsLayoutInput,
+  previous?: InsightsLayout,
 ): InsightsLayout {
+  // v1.16.13 — a PUT that omits a dimension must KEEP the user's stored
+  // value for it, not reset it to defaults. iOS reorders tiles with a
+  // tiles-only PUT (no `sections` key); without the stored fallback that
+  // PUT silently wipes the user's section customization (and vice-versa
+  // for a section-only PUT against stored tiles). When no stored layout
+  // is supplied we still fall back to defaults so a first-ever PUT on a
+  // single dimension persists a complete blob.
   // v1.8.0 — normalise legacy German tile ids onto canonical English
   // before persisting so the stored blob is always canonical even when
   // the PUT body carried the old ids (the Zod enum accepts both via
@@ -308,18 +316,21 @@ export function serializeInsightsLayout(
   // legacy and canonical id for one tile collapses to a single entry,
   // keeping the dense 0-based order contiguous.
   const seen = new Set<string>();
-  // An omitted `tiles` (a section-only PUT) falls back to the canonical
-  // default tile set so the persisted blob is always complete.
+  // An omitted `tiles` (a section-only PUT) keeps the stored tiles when
+  // available, else falls back to the canonical default tile set so the
+  // persisted blob is always complete.
   if (!layout.tiles) {
     return {
       version: INSIGHTS_LAYOUT_VERSION,
-      sections: serializeInsightsSections(layout.sections),
-      tiles: DEFAULT_INSIGHTS_LAYOUT.tiles.map((t) => ({ ...t })),
+      sections: serializeInsightsSections(layout.sections, previous),
+      tiles: (previous?.tiles ?? DEFAULT_INSIGHTS_LAYOUT.tiles).map((t) => ({
+        ...t,
+      })),
     };
   }
   return {
     version: INSIGHTS_LAYOUT_VERSION,
-    sections: serializeInsightsSections(layout.sections),
+    sections: serializeInsightsSections(layout.sections, previous),
     tiles: layout.tiles
       .slice()
       .sort((a, b) => a.order - b.order)
@@ -340,14 +351,19 @@ export function serializeInsightsLayout(
 /**
  * v1.15.11 — normalise an optional `sections` input: drop unknown ids,
  * dedupe (first occurrence by order wins), re-number to a dense 0-based
- * order. If the input omits `sections` entirely, fill from defaults so a
- * current iOS client PUTting only `tiles` still persists a valid v2 blob.
+ * order. If the input omits `sections` entirely, keep the user's stored
+ * sections when supplied (v1.16.13 — a tiles-only PUT must not wipe the
+ * section customization), else fill from defaults so a first-ever
+ * tiles-only PUT still persists a valid v2 blob.
  */
 function serializeInsightsSections(
   sections: InsightsLayoutInput["sections"],
+  previous?: InsightsLayout,
 ): InsightsSectionConfig[] {
   if (!sections) {
-    return DEFAULT_INSIGHTS_LAYOUT.sections.map((s) => ({ ...s }));
+    return (previous?.sections ?? DEFAULT_INSIGHTS_LAYOUT.sections).map((s) => ({
+      ...s,
+    }));
   }
 
   const knownIds = new Set<string>(INSIGHTS_SECTION_IDS);