Mirror getDefaultMileageRate selection in clone helper

Align cloneCustomUnitWithNewIDs with getDefaultMileageRate's exact
default-rate selection: filter to enabled rates first, sort by index
using CONST.DEFAULT_NUMBER_ID for missing indexes, then take the first.
This guarantees the optimistic clone binds the new customUnitRateID
to the same rate the expense flow will later treat as default — even
in edge cases where source rates have an undefined index.

Author: wildan-m

src/libs/PolicyUtils.ts

@@ -309,12 +309,15 @@ function hasEligibleActiveAdminFromWorkspaces(policies: OnyxCollection<Policy> |
 
 function cloneCustomUnitWithNewIDs(unit: CustomUnit, newCustomUnitID: string, newDefaultRateID?: string): CustomUnit {
     if (newDefaultRateID) {
-        // The server-side DUPLICATE_POLICY assigns newDefaultRateID to the source's default rate
-        // (lowest index, matching getDefaultMileageRate). Mirror that here so the optimistic data
-        // doesn't get overwritten when the server response merges. Other source rates get fresh
-        // server IDs, so we drop them from the optimistic state to avoid stale duplicates.
-        const sortedRates = Object.values(unit.rates).sort((a, b) => (a.index ?? Number.MAX_SAFE_INTEGER) - (b.index ?? Number.MAX_SAFE_INTEGER));
-        const defaultRate = sortedRates.find((rate) => rate.enabled !== false) ?? sortedRates.at(0);
+        // The server-side DUPLICATE_POLICY assigns newDefaultRateID to the source's default rate.
+        // Mirror getDefaultMileageRate's selection (enabled rates, sorted by index with
+        // CONST.DEFAULT_NUMBER_ID for missing indexes) so the optimistic clone aligns with the
+        // rate the expense flow will later treat as default. Other source rates get fresh server
+        // IDs, so we drop them from the optimistic state to avoid stale duplicates.
+        const defaultRate = Object.values(unit.rates)
+            .filter((rate) => rate.enabled !== false)
+            .sort((a, b) => (a.index ?? CONST.DEFAULT_NUMBER_ID) - (b.index ?? CONST.DEFAULT_NUMBER_ID))
+            .at(0);
         return {
             ...unit,
             customUnitID: newCustomUnitID,

tests/unit/PolicyUtilsTest.ts

@@ -386,7 +386,7 @@ describe('PolicyUtils', () => {
             });
         });
 
-        it('falls back to the first rate when no enabled rate exists', () => {
+        it('drops all rates when no enabled rate exists', () => {
             const distanceUnitAllDisabled = {
                 customUnitID: 'srcDist',
                 name: CONST.CUSTOM_UNITS.NAME_DISTANCE,
@@ -400,9 +400,36 @@ describe('PolicyUtils', () => {
                 ...createRandomPolicy(0),
                 customUnits: {[distanceUnitAllDisabled.customUnitID]: distanceUnitAllDisabled},
             };
-            const result = getCustomUnitsForDuplication(policyAllDisabled, true, false, {distanceCustomUnitID: 'newDist', perDiemCustomUnitID: 'newPerDiem', customUnitRateID: 'newRate'});
+            const result = getCustomUnitsForDuplication(policyAllDisabled, true, false, {
+                distanceCustomUnitID: 'newDist',
+                perDiemCustomUnitID: 'newPerDiem',
+                customUnitRateID: 'newRate',
+            });
+            expect(result?.newDist.rates).toEqual({});
+        });
+
+        it('treats missing index as 0 when picking the default rate', () => {
+            const distanceUnitWithMissingIndex = {
+                customUnitID: 'srcDist',
+                name: CONST.CUSTOM_UNITS.NAME_DISTANCE,
+                enabled: true,
+                attributes: {unit: CONST.CUSTOM_UNITS.DISTANCE_UNIT_MILES},
+                rates: {
+                    rateB: {customUnitRateID: 'rateB', name: 'Indexed Rate', rate: 100, currency: 'USD', enabled: true, index: 1},
+                    rateA: {customUnitRateID: 'rateA', name: 'No-Index Rate', rate: 70, currency: 'USD', enabled: true},
+                },
+            };
+            const policyWithMissingIndex: Policy = {
+                ...createRandomPolicy(0),
+                customUnits: {[distanceUnitWithMissingIndex.customUnitID]: distanceUnitWithMissingIndex},
+            };
+            const result = getCustomUnitsForDuplication(policyWithMissingIndex, true, false, {
+                distanceCustomUnitID: 'newDist',
+                perDiemCustomUnitID: 'newPerDiem',
+                customUnitRateID: 'newRate',
+            });
             expect(result?.newDist.rates).toEqual({
-                newRate: {customUnitRateID: 'newRate', name: 'Disabled', rate: 50, currency: 'USD', enabled: false, index: 0},
+                newRate: {customUnitRateID: 'newRate', name: 'No-Index Rate', rate: 70, currency: 'USD', enabled: true},
             });
         });
     });