Does anyone have a spreadsheet, or any kind of data that can put these rumours to rest?
1) Any of the 'big boss' NMs (aka Pazuzu, Louhi, Penny etc) require no other NMs to be active in order to spawn.
2) Cassie requires many of the other NMs to be popped and completed to have greater chance of spawning.
Both cases are weird rumours that started getting passed around in the last few months leading up to Shadowbringers launch, and they still persist to this day. It's frustrating that people keep touting these without a shred of evidence to back them up other than "personal experience." For which I have plenty of my own that proves these wrong.
For the first point. In Pagos and Pyros, there's bunny fates that are up permanently that no one ever does. Despite these, Penny and Louhi will spawn with no issue. I've also personally seen 2 other NM fates active in Pyros when we popped Penny.
For the second.... this makes no sense. As long as the entire map is active in prepping Cassie, she will spawn. Failure cases seem to have always been either A; bad rng (usually from starting late in the window) or B; Not enough people present to farm the mobs.
In the case of B, believing in the second point is what's going to guarantee that failure. I had half the map insistent on running around popping every other NM while people were working on Cassie. Of course if people believe it, go and pop other NMs, and then go to Cassie when other people were working on her, it's going to look like it's true. It also seems to fly in the face of the devs who stated that they never intended for players to farm NMs through trains.
I'd love to be proven wrong, I really would. But as far as I can tell it looks like someone came through and trolled everyone by getting them to hold anecdotal evidence and confirmation bias as hard fact. This is like the weird rituals and prayers people make up to convince themselves their lottery ticket became 'more lucky.' Rng is just rng. It pops or it doesn't.