diff --git a/app_dart/lib/src/request_handlers/flaky_handler_utils.dart b/app_dart/lib/src/request_handlers/flaky_handler_utils.dart
index 795b389e4..19448034a 100644
--- a/app_dart/lib/src/request_handlers/flaky_handler_utils.dart
+++ b/app_dart/lib/src/request_handlers/flaky_handler_utils.dart
@@ -145,11 +145,8 @@ class IssueUpdateBuilder {
   List<String> get issueLabels {
     final List<String> existingLabels = existingIssue.labels.map<String>((IssueLabel label) => label.name).toList();
     // Update the priority.
-    if (!existingLabels.contains(kP0Label) && !isBelow) {
+    if (!isBelow && !_containsPriorityLabel(existingLabels)) {
       existingLabels.add(kP0Label);
-      existingLabels.remove(kP1Label);
-      existingLabels.remove(kP2Label);
-      existingLabels.remove(kP3Label);
     }
     return existingLabels;
   }
@@ -170,6 +167,16 @@ ${_issueBuilderLink(statistic.name)}
     }
     return result;
   }
+
+  bool _containsPriorityLabel(List<String> labels) {
+    for (final String label in labels) {
+      if (label == kP0Label || label == kP1Label || label == kP2Label || label == kP3Label) {
+        return true;
+      }
+    }
+
+    return false;
+  }
 }
 
 /// A builder to build the pull request title and body for marking test flaky
diff --git a/app_dart/test/request_handlers/update_existing_flaky_issues_test.dart b/app_dart/test/request_handlers/update_existing_flaky_issues_test.dart
index 77c8d2ddb..9ba9434f2 100644
--- a/app_dart/test/request_handlers/update_existing_flaky_issues_test.dart
+++ b/app_dart/test/request_handlers/update_existing_flaky_issues_test.dart
@@ -90,7 +90,7 @@ void main() {
       const int existingIssueNumber = 1234;
       final List<IssueLabel> existingLabels = <IssueLabel>[
         IssueLabel(name: 'some random label'),
-        IssueLabel(name: 'P2'),
+        IssueLabel(name: 'P0'),
       ];
       // When queries flaky data from BigQuery.
       when(mockBigqueryService.listBuilderStatistic(kBigQueryProjectId)).thenAnswer((Invocation invocation) {
@@ -155,7 +155,7 @@ void main() {
       expect(result['Status'], 'success');
     });
 
-    test('Add only one comment on existing issue when a builder has been marked as unflaky', () async {
+    test('Respects existing lower priority label', () async {
       const int existingIssueNumber = 1234;
       final List<IssueLabel> existingLabels = <IssueLabel>[
         IssueLabel(name: 'some random label'),
@@ -165,6 +165,75 @@ void main() {
       when(mockBigqueryService.listBuilderStatistic(kBigQueryProjectId)).thenAnswer((Invocation invocation) {
         return Future<List<BuilderStatistic>>.value(semanticsIntegrationTestResponse);
       });
+      when(mockBigqueryService.listBuilderStatistic(kBigQueryProjectId, bucket: 'staging'))
+          .thenAnswer((Invocation invocation) {
+        return Future<List<BuilderStatistic>>.value(stagingCiyamlTestResponse);
+      });
+      // when gets existing flaky issues.
+      when(mockIssuesService.listByRepo(captureAny, state: captureAnyNamed('state'), labels: captureAnyNamed('labels')))
+          .thenAnswer((Invocation invocation) {
+        return Stream<Issue>.fromIterable(<Issue>[
+          Issue(
+            assignee: User(login: 'some dude'),
+            number: existingIssueNumber,
+            state: 'open',
+            labels: existingLabels,
+            title: expectedSemanticsIntegrationTestResponseTitle,
+            body: expectedSemanticsIntegrationTestResponseBody,
+            createdAt:
+                DateTime.now().subtract(const Duration(days: UpdateExistingFlakyIssue.kFreshPeriodForOpenFlake + 1)),
+          ),
+        ]);
+      });
+      // when firing github request.
+      // This is for replacing labels.
+      when(
+        mockGitHubClient.request(
+          captureAny,
+          captureAny,
+          body: captureAnyNamed('body'),
+        ),
+      ).thenAnswer((Invocation invocation) {
+        return Future<Response>.value(Response('[]', 200));
+      });
+      final Map<String, dynamic> result = await utf8.decoder
+          .bind((await tester.get<Body>(handler)).serialize() as Stream<List<int>>)
+          .transform(json.decoder)
+          .single as Map<String, dynamic>;
+
+      // Verify comment is created correctly.
+      List<dynamic> captured = verify(mockIssuesService.createComment(captureAny, captureAny, captureAny)).captured;
+      expect(captured.length, 3);
+      expect(captured[0].toString(), Config.flutterSlug.toString());
+      expect(captured[1], existingIssueNumber);
+      expect(captured[2], expectedSemanticsIntegrationTestIssueComment);
+
+      // Verify labels are applied correctly.
+      captured = verify(
+        mockGitHubClient.request(
+          captureAny,
+          captureAny,
+          body: captureAnyNamed('body'),
+        ),
+      ).captured;
+      expect(captured.length, 3);
+      expect(captured[0].toString(), 'PUT');
+      expect(captured[1], '/repos/${Config.flutterSlug.fullName}/issues/$existingIssueNumber/labels');
+      expect(captured[2], GitHubJson.encode(<String>['some random label', 'P2']));
+
+      expect(result['Status'], 'success');
+    });
+
+    test('Add only one comment on existing issue when a builder has been marked as unflaky', () async {
+      const int existingIssueNumber = 1234;
+      final List<IssueLabel> existingLabels = <IssueLabel>[
+        IssueLabel(name: 'some random label'),
+        IssueLabel(name: 'P0'),
+      ];
+      // When queries flaky data from BigQuery.
+      when(mockBigqueryService.listBuilderStatistic(kBigQueryProjectId)).thenAnswer((Invocation invocation) {
+        return Future<List<BuilderStatistic>>.value(semanticsIntegrationTestResponse);
+      });
       when(mockBigqueryService.listBuilderStatistic(kBigQueryProjectId, bucket: 'staging'))
           .thenAnswer((Invocation invocation) {
         return Future<List<BuilderStatistic>>.value(stagingSameBuilderSemanticsIntegrationTestResponse);