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.
Only the Mark1 sync marker is deleted. This is the behavior of Sync Markers created through the context menu.
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.