Using the delete key or context menu to delete a single sync marker created by an Animation Modifier deletes all sync markers created by the Animation Modifier.
If all the markers are deleted, and the action is undone then a single marker is deleted it behaves as expected.
If sync markers are created then deleted through the context menu in the Notifies Track then a single Marker is deleted the rest remain.
Expected Result:
Only the Mark1 sync marker is deleted. This is the behavior of Sync Markers created through the context menu.
Actual Results:
All the sync markers are deleted.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-139286 in the post.