Final Steps of Faith question

I know this is super old content, but I've been suddenly getting this trial a lot lately in roulettes for some reason. I'm terrible at remembering fights in old content, so usually I just wing it and everything turns out okay.

During the final phase of the fight, a random player gets a stack marker. Pretty straightforward. Everyone rushes in to stack on the player. But you're supposed to stay stacked for a few seconds after the initial burst because Nidhogg unleashes like three or four more blasts.

I notice sometimes after you're done stacking, he turns to individual players and one-shots them. There is no telegraph, no visual warning of any kind. He just randomly turns and decimates half the raid. That's not an exaggeration. That actually happened the other night where he turned and killed the healer and two DPS, one of them being me.

I cannot remember for the life of me how I always avoided this in the past? Videos don't say anything about it. I doubt it's a new mechanic. The only thing I can think of is that maybe we didn't stay stacked long enough? Or maybe the tanks were running it for the first time and losing aggro?

This has happened twice now and it's kind of embarrassing that I'm dying in this fight. Clearing Eden no problem. Killing Nidhogg? Huge, massive, problem. :p

Either the stack split early or the MT stayed stacked too long and the boss used their fire breath cleave.

This should only happen when people don't stack. Then Niddhog just goes and one-shots random dudes. So maybe people run out of the stack too early?

If the initial target of the stackup dies, it picks a new target for the next stackup blast. If that person isn't properly stacked, they will die alone instantly and then it picks a different target again for the next stackup blast, and so on. All these separated people rapidly die in a row because they're supposed to be all together. If the whole party is properly stacked together and stays together through every blast, then every blast will be on the same position and not 1-shotting anybody.

I don't know if nidd has a TB cleave right after akh morn. That would be kinda mean if so, but I haven't done the fight in a while