PC NA. I’ve done this probably hundreds of times and it was never a problem before. And yeah, when the health bar wasn’t moving I knew I had the wrong one but never had to resort to that before.
They do cycle immunity, and the transparency isn't exactly obvious. Watch the target frame health bar. If it isn't moving, try an different target.
Just following up here, we do have a bug info for this and the team is working toward a fix. We'll have more information to share once the fix is available.