Repro steps:
1. Ensure player has 20 health
2. Equip armor or hold an item with a modifier that reduces maxHealth below 20
3. Drink an instant health potion or use /effect <player> 6

The player will lurch as if taking damage, but will not lose any health. If maxHealth is lower than the amount of health given, the player will see the "You died!" screen, even though they are not dead. If they click Respawn, they will return to the game without respawning, but the player's body will be colored red and they will be leaning sideways and wobbling. This is not visible to other players and will go away if the player reconnects.

This only happens the first time instant health is applied after each time the modifier is applied. After this, instant health seems to work normally.