diff --git a/server/src/services/settingsService.ts b/server/src/services/settingsService.ts
index d58f53495..7f0580021 100644
--- a/server/src/services/settingsService.ts
+++ b/server/src/services/settingsService.ts
@@ -36,6 +36,8 @@ export const DEFAULTABLE_USER_SETTING_KEYS = [
 
 type DefaultableKey = typeof DEFAULTABLE_USER_SETTING_KEYS[number];
 
+const DEFAULTABLE_USER_SETTING_KEY_SET = new Set<string>(DEFAULTABLE_USER_SETTING_KEYS);
+
 const VALID_VALUES: Partial<Record<DefaultableKey, unknown[]>> = {
   temperature_unit: ['fahrenheit', 'celsius'],
   distance_unit: ['metric', 'imperial'],
@@ -132,8 +134,24 @@ export function getUserSettings(userId: number): Record<string, unknown> {
     }
   }
 
-  // Admin defaults fill in only for keys the user hasn't explicitly set
-  return { ...adminDefaults, ...userSettings };
+  // Admin defaults fill in for keys the user hasn't explicitly set. For a
+  // defaultable key an *empty* user value counts as "not set": the client's
+  // Settings save always writes every field (a blank Mapbox token included), so
+  // an empty string would otherwise shadow the admin/system-wide default and the
+  // user could never fall back to it — even after clearing their own value
+  // (#1634). Non-defaultable keys keep their exact stored value.
+  const merged: Record<string, unknown> = { ...adminDefaults };
+  for (const [key, value] of Object.entries(userSettings)) {
+    if (
+      DEFAULTABLE_USER_SETTING_KEY_SET.has(key) &&
+      (value === '' || value === null || value === undefined) &&
+      key in adminDefaults
+    ) {
+      continue; // empty user value → keep the admin default
+    }
+    merged[key] = value;
+  }
+  return merged;
 }
 
 function serializeValue(key: string, value: unknown): string {
diff --git a/server/tests/unit/services/settingsService.test.ts b/server/tests/unit/services/settingsService.test.ts
index d7e9f1da1..92f92d2a7 100644
--- a/server/tests/unit/services/settingsService.test.ts
+++ b/server/tests/unit/services/settingsService.test.ts
@@ -34,6 +34,7 @@ vi.mock('../../../src/config', () => ({
 // Passthrough crypto — value comes back unchanged for most tests
 vi.mock('../../../src/services/apiKeyCrypto', () => ({
   maybe_encrypt_api_key: (v: string) => v,
+  decrypt_api_key: (v: string) => v,
 }));
 
 import { createTables } from '../../../src/db/schema';
@@ -111,6 +112,46 @@ describe('getUserSettings', () => {
     expect(s).toHaveProperty('key_a');
     expect(s).not.toHaveProperty('key_b');
   });
+
+  // Admin "user defaults" fall-through (#1634) — a system-wide Mapbox token must
+  // reach a user who left their own token blank.
+  const setAdminDefault = (settingKey: string, value: string) =>
+    testDb.prepare(
+      "INSERT INTO app_settings (key, value) VALUES (?, ?) ON CONFLICT(key) DO UPDATE SET value = excluded.value",
+    ).run(`default_user_setting_${settingKey}`, value);
+
+  it('SET-SVC-020 — new user with no rows inherits the admin default token', () => {
+    const { user } = createUser(testDb);
+    setAdminDefault('mapbox_access_token', 'pk.admin');
+    expect(getUserSettings(user.id).mapbox_access_token).toBe('pk.admin');
+  });
+
+  it('SET-SVC-021 — an empty user token falls through to the admin default', () => {
+    const { user } = createUser(testDb);
+    setAdminDefault('mapbox_access_token', 'pk.admin');
+    testDb.prepare("INSERT INTO settings (user_id, key, value) VALUES (?, 'mapbox_access_token', '')").run(user.id);
+    expect(getUserSettings(user.id).mapbox_access_token).toBe('pk.admin');
+  });
+
+  it('SET-SVC-022 — a non-empty user token overrides the admin default', () => {
+    const { user } = createUser(testDb);
+    setAdminDefault('mapbox_access_token', 'pk.admin');
+    testDb.prepare("INSERT INTO settings (user_id, key, value) VALUES (?, 'mapbox_access_token', 'pk.user')").run(user.id);
+    expect(getUserSettings(user.id).mapbox_access_token).toBe('pk.user');
+  });
+
+  it('SET-SVC-023 — an empty value with no admin default stays empty (no regression)', () => {
+    const { user } = createUser(testDb);
+    testDb.prepare("INSERT INTO settings (user_id, key, value) VALUES (?, 'mapbox_style', '')").run(user.id);
+    expect(getUserSettings(user.id).mapbox_style).toBe('');
+  });
+
+  it('SET-SVC-024 — a non-defaultable empty value is preserved even against a same-named default', () => {
+    const { user } = createUser(testDb);
+    // 'theme' is not a defaultable key: an empty stored value must be returned as-is.
+    testDb.prepare("INSERT INTO settings (user_id, key, value) VALUES (?, 'theme', '')").run(user.id);
+    expect(getUserSettings(user.id).theme).toBe('');
+  });
 });
 
 // ── upsertSetting ─────────────────────────────────────────────────────────────