Gameplay Effects can hold stale references to Gameplay Tags when the adding component is switched for a different type. The tags are serialised, and their presence can be confirmed with Reference Viewer or with a file editor.
1. Create a GameplayEffect
2. Add an Asset Tag Gameplay Effect Component
2.a. Add any tag to the added tags list
3. Compile and save the asset
4. Reference Viewer will now should the reference
5. Switch the component type to anything else
5.a. Blank Additional Effects Gameplay Effect Component was used in testing
6. Compile and save the asset
7. Reference Viewer will still show the reference, despite is seemingly being gone
1. Add a new blank Asset Tag Gameplay Effect Component
2. Compile and save
3. Reference Viewer will no longer show references
4. Delete the blank Asset Tag Gameplay Effect Component
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-203558 in the post.