I created a helmet with custom nbt to give the user +80 max health, but when I tried it in survival mode. After a second the new hearts disappeared, so to test whether the bug was just visual I gave myself poison 1 for 1 second. That should have only given me one damage, but instead I died and when I respawned, I looked like this (see attached screenshot) and I was not dead or dying, and I still had only one row of hearts.