Steps to reproduce:

1. Create a new world with commands enabled and in Hard difficulty.
2. Go into survival mode.
3. Give yourself and equip a Totem of Undying.
4. Run this command: /damage @s 300000000000000000000000000000000000000 minecraft:mob_attack by @e[type=minecraft:zombie,limit=1,sort=nearest]
   If the command fails, find or summon a zombie then try again.
5. Run this command: /data get entity @s AbsorptionAmount
   [Player] has the following entity data: NaNf
6. Take damage again.
7. Your screen will appear to constantly shake and your health bar will show as if you had 0 HP, but you still remain alive. You can still move around, break blocks, and punch entities, but you can't pick up items.
8. Run this command: /data get entity @s Health
   [Player] has the following entity data: NaNf

Once the bug occurs for a player, it can only be fixed by setting their Health and AbsorptionAmount to a valid number using external tools or mods; the /kill command, which normally instantly kills selected entities, won't work on such players.