Under circumstances I am not certain of, the health bar will not regenerate to full health despite a full hunger bar. This is with the naturalRegeneration being set to True and without being affected by any status effects. Using the command /gamerule naturalRegeneration true has no effect. I encountered this problem while raiding End Cities in version 1.21, originally in a modded version but I have verified that the problem persists in Vanilla. It appears to occur when fighting many shulkers at once, namely in the fat_tower_middle and fat_tower_base structures, which contain a high abundance of shulkers. Health appears to regenerate as normal when only fighting a few mobs at once. This is on a vanilla multiplayer server, with no datapacks installed. Disabling all my resource packs also has no effect.

Steps to reproduce

1. In the End dimension, locate an End city and find its fat_tower_middle and fat_tower_base segments, if it has them. It may help to be in a multiplayer server.
2. In survival mode, fight the shulkers.
3. Health may not regenerate despite a full hunger bar.

I have not tested these steps to reproduce, since I encountered this issue during normal gameplay they are simply the conditions under which the issue occurred for me and are therefore the conditions I consider to be mostly likely to reproduce the possible bug. I believe that eating causes the natural regeneration of hearts to resume, which is possible with golden apples if the hunger bar is full.

If there is something I may be missing here, please let me know. I have not encountered this problem before and I am not aware of any legitimate reason for it to occur.

Update: By deliberately consuming some hunger points by jumping in place, the natural heart regeneration has resumed. I am wondering if this is just a slight quirk of the regeneration system.