fix(ecs-patterns): update feature flag documentation for uniqueTargetGroupId

Signed-off-by: puretension <rlrlfhtm5@gmail.com>

Author: puretension

packages/aws-cdk-lib/cx-api/FEATURE_FLAGS.md

@@ -107,6 +107,7 @@ Flags come in three types:
 | [@aws-cdk/core:explicitStackTags](#aws-cdkcoreexplicitstacktags) | When enabled, stack tags need to be assigned explicitly on a Stack. | 2.205.0 | new default |
 | [@aws-cdk/aws-signer:signingProfileNamePassedToCfn](#aws-cdkaws-signersigningprofilenamepassedtocfn) | Pass signingProfileName to CfnSigningProfile | 2.212.0 | fix |
 | [@aws-cdk/aws-ecs-patterns:secGroupsDisablesImplicitOpenListener](#aws-cdkaws-ecs-patternssecgroupsdisablesimplicitopenlistener) | Disable implicit openListener when custom security groups are provided | 2.214.0 | new default |
+| [@aws-cdk/aws-ecs-patterns:uniqueTargetGroupId](#aws-cdkaws-ecs-patternsuniquetargetgroupid) | When enabled, ECS patterns will generate unique target group IDs to prevent conflicts during load balancer replacement | V2NEXT | fix |
 
 <!-- END table -->
 
@@ -197,7 +198,8 @@ The following json shows the current recommended set of flags, as `cdk init` wou
     "@aws-cdk/s3-notifications:addS3TrustKeyPolicyForSnsSubscriptions": true,
     "@aws-cdk/aws-ec2:requirePrivateSubnetsForEgressOnlyInternetGateway": true,
     "@aws-cdk/aws-s3:publicAccessBlockedByDefault": true,
-    "@aws-cdk/aws-lambda:useCdkManagedLogGroup": true
+    "@aws-cdk/aws-lambda:useCdkManagedLogGroup": true,
+    "@aws-cdk/aws-ecs-patterns:uniqueTargetGroupId": true
   }
 }
 ```
@@ -2277,4 +2279,26 @@ override this behavior.
 **Compatibility with old behavior:** You can pass `openListener: true` explicitly to maintain the old behavior.
 
 
+### @aws-cdk/aws-ecs-patterns:uniqueTargetGroupId
+
+*When enabled, ECS patterns will generate unique target group IDs to prevent conflicts during load balancer replacement*
+
+Flag type: Backwards incompatible bugfix
+
+When this feature flag is enabled, ECS patterns will generate unique target group IDs that include
+both the load balancer type (public/private) and load balancer name. This prevents CloudFormation
+conflicts when switching between public and private load balancers or when changing load balancer names.
+
+Without this flag, target groups use generic IDs like 'ECS' which can cause conflicts when the
+underlying load balancer is replaced due to changes in internetFacing or loadBalancerName properties.
+
+This is a breaking change as it will cause target group replacement when the flag is enabled.
+
+
+| Since | Unset behaves like | Recommended value |
+| ----- | ----- | ----- |
+| (not in v1) |  |  |
+| V2NEXT | `false` | `true` |
+
+
 <!-- END details -->

packages/aws-cdk-lib/recommended-feature-flags.json

@@ -78,5 +78,6 @@
   "@aws-cdk/s3-notifications:addS3TrustKeyPolicyForSnsSubscriptions": true,
   "@aws-cdk/aws-ec2:requirePrivateSubnetsForEgressOnlyInternetGateway": true,
   "@aws-cdk/aws-s3:publicAccessBlockedByDefault": true,
-  "@aws-cdk/aws-lambda:useCdkManagedLogGroup": true
+  "@aws-cdk/aws-lambda:useCdkManagedLogGroup": true,
+  "@aws-cdk/aws-ecs-patterns:uniqueTargetGroupId": true
 }