if u have armor with negative maxhealth on it doesnt update (https://mojang.atlassian.net/browse/MC-28127) but if you want to update this with /effect and appy the instant health effect you see a death message but it is not displayed in the chat. if you click respawn you dont respawn, you can play on futher. for you the player is walking sideways and red. like the player is dieing. for other players it you are walking normal rejoining the world or dieing will solve this visual bug