If a mob (only tried with zombies but I assume all) is teleported to an unloaded chunk, it goes invisible. It is still there and reappears when I relog. Whist invisible, I cannot attack it either.

I further tried giving it the invisibility effect for 3 seconds to see if it would "reboot" its visibility and it did not.

I gave it armor using /replaceitem and that did not render until I relogged.