Only fall back to default tax in getTaxName when tax tracking is enabled

When a user deletes tax from an expense (with taxes disabled), the API
returns taxCode: null which Onyx strips to undefined. getTaxName then
falls back to defaultTaxCode via the || operator and displays the
default tax rate instead of showing empty.

This uses policy.tax.trackingEnabled to decide the fallback: if taxes
are enabled and taxCode is falsy, fall back to the default (fixing
issue 85729). If taxes are disabled and taxCode is falsy, return
undefined so the display shows empty (fixing issue 87667).

Co-authored-by: Vinh Hoang <hoangzinh@users.noreply.github.com>

Author: MelvinBot

src/libs/TransactionUtils/index.ts

@@ -2141,9 +2141,12 @@ function getWorkspaceTaxesSettingsName(policy: OnyxEntry<Policy>, taxCode: strin
 function getTaxName(policy: OnyxEntry<Policy>, transaction: OnyxEntry<Transaction>, shouldFallbackToValue = false) {
     const defaultTaxCode = getDefaultTaxCode(policy, transaction);
 
-    // transaction?.taxCode may be an empty string
+    // Only fall back to the default tax code when tax tracking is enabled on the policy.
+    // When taxes are disabled and the user deletes a tax, taxCode becomes undefined (the API returns null, which Onyx strips).
+    // Without this check, getTaxName would fall back to defaultTaxCode and display the default tax rate instead of showing empty.
     // eslint-disable-next-line @typescript-eslint/prefer-nullish-coalescing
-    const taxRate = Object.values(transformedTaxRates(policy, transaction)).find((rate) => rate.code === (transaction?.taxCode || defaultTaxCode));
+    const effectiveTaxCode = transaction?.taxCode || (policy?.tax?.trackingEnabled ? defaultTaxCode : undefined);
+    const taxRate = effectiveTaxCode ? Object.values(transformedTaxRates(policy, transaction)).find((rate) => rate.code === effectiveTaxCode) : undefined;
 
     if (shouldFallbackToValue && transaction?.taxValue !== undefined && taxRate?.value !== transaction?.taxValue) {
         return transaction?.taxValue;

tests/unit/TransactionUtilsTest.ts

@@ -1829,6 +1829,7 @@ describe('TransactionUtils', () => {
     describe('getTaxRateTitle', () => {
         const policy: Policy = {
             ...createRandomPolicy(0),
+            tax: {trackingEnabled: true},
             taxRates: CONST.DEFAULT_TAX,
         };