I encountered this with the 1.21.20 update and have some observations:
This seems to apply only to regeneration with food.
Switching to peaceful difficulty allows HP to reach 20, and will remain 20 after switching back to a non-peaceful difficulty. However, taking damage and regaining health by eating seems to revert to the 19 HP ceiling.
Having the regeneration effect gains full health.
Drinking a health potion also can bring health up to full.
I've tested this on both an Android 12 device and on Windows 11.
Estragon
I encountered this with the 1.21.20 update and have some observations:
This seems to apply only to regeneration with food.
Switching to peaceful difficulty allows HP to reach 20, and will remain 20 after switching back to a non-peaceful difficulty. However, taking damage and regaining health by eating seems to revert to the 19 HP ceiling.
Having the regeneration effect gains full health.
Drinking a health potion also can bring health up to full.
I've tested this on both an Android 12 device and on Windows 11.