We had the sorta same on official as we killed a sleeper, but he died trough poison pike, he was later stood in T-pose and invulnerable.
Something like this happened to our tribe members too when they log out starving, the log is every few seconds posting that he died through hunger as example. Same with bleed, posion, heat and cold…
Some ppl do that on purpose (burning themselves, starving etc) so they can still come back and get their loot when they relog later, coz only relog stops this invulnerability.