You cannot remove the amulet unless you have max health healed, otherwise I'm assuming it removes the taken damage fraction to max health and applies it to the player, removing more hearts than the player has, just an assumption. When taking damage, everything is fine, until you remove/unequip the amulet and the player gets instantly killed.

This is intended, imagine having 40 hp taking 25 hearts worth damage to be able to remove the charm and be back at 20 would be exploitable using the charm as a free heal, but what I do is math the value thus taking it off would leave the player at -5 hearts which would kill the player