1.21.20 PC, survival - have the same problem.
Regen potion healed up to 10 hearts. I haven't taken damage so I cannot confirm if it gets stuck again at 9 1/2 hearts.
I had this happen this afternoon. I've brought tamed wolves through Nether portals many times. This is the first time I lost one.
I was playing with a friend in a locally hosted world. She has lost a cat before.
1.21.20 PC, survival - have the same problem.
Regen potion healed up to 10 hearts. I haven't taken damage so I cannot confirm if it gets stuck again at 9 1/2 hearts.