diff --git a/CHANGELOG.md b/CHANGELOG.md
index a2032c41cbd..eceb710eb85 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -116,6 +116,7 @@
 * [BUGFIX] Alertmanager: Tighten per-tenant config validation to reject additional file-based settings. #7767
 * [BUGFIX] Querier: Fix panic (`index out of range [-1]`) in the active request tracker when truncating a `match[]`/`query` value made entirely of invalid UTF-8 continuation bytes. The backwards scan for a rune boundary now stops at index 0 instead of underflowing. #7743
 * [BUGFIX] Config: Fix CSV-list flags/YAML fields (e.g. `-compactor.enabled-tenants`) treating an explicitly empty string as a one-element list containing an empty tenant name instead of an empty list. #7714
+* [BUGFIX] Tenant Federation: Fix regex tenant federation dropping tenants when `-blocks-storage.users-scanner.cache-ttl` is set. The regex resolver sorted the user list returned by the users scanner in place, corrupting the scanner cache and progressively losing tenants on every sync until the cache expired. #7812
 
 ## 1.21.1 2026-06-04
 
diff --git a/pkg/querier/tenantfederation/regex_resolver.go b/pkg/querier/tenantfederation/regex_resolver.go
index 9fb2bc33aed..7d60d7dc7d6 100644
--- a/pkg/querier/tenantfederation/regex_resolver.go
+++ b/pkg/querier/tenantfederation/regex_resolver.go
@@ -128,7 +128,9 @@ func (r *RegexResolver) running(ctx context.Context) error {
 				continue
 			}
 
-			newUsers := append(active, deleting...)
+			newUsers := make([]string, 0, len(active)+len(deleting))
+			newUsers = append(newUsers, active...)
+			newUsers = append(newUsers, deleting...)
 			sort.Strings(newUsers)
 
 			r.Lock()
diff --git a/pkg/querier/tenantfederation/regex_resolver_test.go b/pkg/querier/tenantfederation/regex_resolver_test.go
index 92285e835eb..542b37b202b 100644
--- a/pkg/querier/tenantfederation/regex_resolver_test.go
+++ b/pkg/querier/tenantfederation/regex_resolver_test.go
@@ -266,6 +266,59 @@ func Test_RegexResolver_Cache(t *testing.T) {
 	}
 }
 
+func Test_RegexResolver_DoesNotMutateScannerResult(t *testing.T) {
+	reg := prometheus.NewRegistry()
+
+	activeTenants := []string{"b-1", "b-2", "b-3"}
+	deletingTenant := "a-del"
+	expected := []string{"a-del", "b-1", "b-2", "b-3"}
+
+	bucketClient := &bucket.ClientMock{}
+	bucketClient.MockIter("", append([]string{deletingTenant}, activeTenants...), nil)
+	bucketClient.MockIter("__markers__", []string{"__markers__/" + deletingTenant + "/"}, nil)
+	bucketClient.MockExists(users.GetGlobalDeletionMarkPath(deletingTenant), true, nil)
+	for _, tenant := range activeTenants {
+		bucketClient.MockExists(users.GetGlobalDeletionMarkPath(tenant), false, nil)
+		bucketClient.MockExists(users.GetLocalDeletionMarkPath(tenant), false, nil)
+	}
+
+	bucketClientFactory := func(ctx context.Context) (objstore.InstrumentedBucket, error) {
+		return bucketClient, nil
+	}
+
+	// Caching makes the users scanner return the very same slices on every scan, so
+	// mutating them in place would corrupt the result of all the following scans.
+	usersScannerConfig := users.UsersScannerConfig{Strategy: users.UserScanStrategyList, CacheTTL: time.Hour}
+	tenantFederationConfig := Config{UserSyncInterval: 50 * time.Millisecond, MaxTenant: 0, RegexCacheSize: 10}
+	regexResolver, err := NewRegexResolver(usersScannerConfig, tenantFederationConfig, reg, bucketClientFactory, log.NewNopLogger())
+	require.NoError(t, err)
+
+	require.NoError(t, services.StartAndAwaitRunning(context.Background(), regexResolver))
+	defer services.StopAndAwaitTerminated(context.Background(), regexResolver) //nolint:errcheck
+
+	// Wait for the first scan.
+	test.Poll(t, 10*time.Second, true, func() any {
+		return testutil.ToFloat64(regexResolver.lastUpdateUserRun) > 0
+	})
+	firstRun := testutil.ToFloat64(regexResolver.lastUpdateUserRun)
+
+	// Wait for several more scans. The corruption only shows up from the second scan
+	// on, once the scanner starts returning the previously mutated slices.
+	test.Poll(t, 10*time.Second, true, func() any {
+		return testutil.ToFloat64(regexResolver.lastUpdateUserRun) > firstRun
+	})
+
+	regexResolver.RLock()
+	knownUsers := append([]string(nil), regexResolver.knownUsers...)
+	regexResolver.RUnlock()
+	require.Equal(t, expected, knownUsers)
+
+	ctx := user.InjectOrgID(context.Background(), "b-.+")
+	orgIDs, err := regexResolver.TenantIDs(ctx)
+	require.NoError(t, err)
+	require.Equal(t, activeTenants, orgIDs)
+}
+
 func Test_RegexResolver_CacheInvalidation(t *testing.T) {
 	reg := prometheus.NewRegistry()
 	initialTenants := []string{"user-1", "user-2"}