I have only done Ny’alotha in Mythic for the mount farm during this current event. I also never came here before that event either. The boss [The Prophet Skitara] enters a phase where it clones itself and becomes invisibile, but cant still take damage (from at least DoT’s) such as those from my Assassination Rogue/Warlock/Death Knight.
When/If she dies while hidden/invisible, she will not be interactable and loot cannot be taken from her. Please note that she does have her loot sent to you via mail, but any quest/specific loot on kill items she may carry like currency is not sent, so there is a net loss.
Reloading does not solve this.