It seems that if you place the wall just right you are able to animation lock the targeting animation for the guardian. Wasn't able to reproduce, tried for about 10 minutes with the same/similar circumstances:
- Full size characters and small characters
- Minions
- Different angles
- Same tier...
Celeste Ultimate able to be held indefinitely if left mouse click is held at any point during the casting animation. Assuming this is a bug. Already posted video in discord under username "deathluca", decided it can't hurt to post here too.