When you have an item which modifies your max hp (at least by 20) and you switch to another item while fully regenerated, next damage will make death screen show up. You are not actually dead, no death message shows up, just the death screen. Your player body will be stuck in death animation (even in the inventory window). You can still take damage and die for real. If you click respawn, death screen closes and you normally continue.

Steps to replicate:

1. Give yourself a max health altering item, e.g.

   /give @p minecraft:stone{AttributeModifiers:[{AttributeName:"generic.maxHealth",Name:"generic.maxHealth",Amount:20,Operation:0,UUIDLeast:978024,UUIDMost:531064}]} 1
   

   It has to add at least 20 hp (one line), otherwise it won't work. If it adds more than 20 hp, it is not necessary to regenerate further than 20 hp.

1. Select the item in hotbar and regenerate above 40 hp (two lines of hearts).
2. Select another item in hotbar.
3. Let yourself be hit by a mob - I tested it with skeletons (arrows), zombies, spiders, creepers, anvils. Fall damage doesn't show the death screen up but still instantly gets rid of your additional hp. It needs more testing for more damage types.
4. Death screen will show up and item added hp (above normal hp - 20+ hp) will be instantly lost (even if the damage was much smaller). Your body will be stuck in death animation (also in inventory). You can still take damage and knockback.
5. After clicking "Respawn", death screen will disappear and everything works normally, except of your body being stuck in death animation.

I tested it on a new void world and it worked every single time. It is fixed after relog.