If a player is healed about the same time they are killed, whether it be a command block with effects (fast clock) (edit)sorry, didnt know that would link:, /heal, or even harder to reproduce, throwing a heal potion, before a player dies, they will appear in chat as they died to what cause, and seemingly, the player that barely lived will see themself as dead/ red, twitching, horizontal. other players usually see the player as normal/not there. i have done this in ssp, new launcher, no mods.
Ill link a few screenshots of moments after finding it. i searched this, found no duplicates i can see.
MC-19690 Reducing maxHealth / max_health can cause fake death