I’m not sure what changed with the legacy changes for BFA raids since patch, but N’zoth LFR fight is bugged. You get him to his second shell phase to go down right before you kill and the portal to take you down doesn’t activate it. It will disappear and then N’zoth just kills you with nothing you can do about it. It makes it impossible to farm for azerite armor for Timewalking or for mog. Please fix this.
That’s called Fettered Mind. It is only supposed to happen on heroic or above. As of this week, it is erroneously happening on LFR level, and possibly Normal too. It is a bug, and I reported it to Blizzard. When it is fixed, LFR N’Zoth should be soloable again. Fettered Mind was/is why N’Zoth can’t be solo’d on heroic or mythic.