I was invincible during a grapple. Everybody was shooting and swinging weapon at me but I couldn't get hurt from any handheld weapon (didn't try canons or fall). Could't hurt anyone either. I was the last man alive on my crew for a while but as soon as my alive respawned (timer) I lost my immunity and was killed. It's worth mentioning that I hadn't died since the beginning of the game.
[DEV]: Unable to recreate so no leads