The bug triggers by taken more damage than 1/2 of your final heart if you set it to peaceful than set it to easy or higher it will regenerate normally, if you take damage more than 1/2 on easy or higher it won't regenerate last 1/2 heart.
I've got the same issue. 1.21.21-PS5
It can be fixed by death or regeneration for now.
It's seems to start when you put armor on.
DOES NOT EFFECT PEACEFUL DIFFICULTY
The bug triggers by taken more damage than 1/2 of your final heart if you set it to peaceful than set it to easy or higher it will regenerate normally, if you take damage more than 1/2 on easy or higher it won't regenerate last 1/2 heart.