Sometimes, some hostile mobs are invisible, but this is not the invisibility effect because armor is also invisible and if I quit the game and restart, the mob won't be invisible...
It did a similar thing with an indestructible ender crystal when the crystal was moved by a flying machine, but i don't know if it is the same bug.
I am in 1.16.2 and in hard difficulty and I observed the bug with a crossbow piglin (it was on fire when it died but the fire wasn't visible too), an armored (iron) skeleton and an iron ingot-wearing zombie (the zombie was used for an iron farm, so it's impossible that mobs affected by the bug are spawning with invisibility)
Unfortunately, I don't know how to reproduce correctly the bug...
I made screenshots of the invisible zombie.