Actually, its not full rng imo.
Ive done a lot of tw, specially since bfa, as i use them for lvling aswell. So many toons went through.
In my experience, it rotates, so i get all dungeons and only when done i get doubles, usually, that is. Its even so little randomized, i often predict accurately which one im going to get.
Not saying what you say is wrong, bc if you fit a group of other players who have not done pos yet, chances are you will get it. So you must have had real bad luck.
i find you only get the same one if you change chars or log out between dungeons…but if you do all five in a row you get all of them…might just be me, but that’s been a consistent since TW started…
I usually get The Nexus and HoL. I’ve had PoS twice. And bear in mind, I got the 5 dungeons completed, then I ground out 1000 badges for the mount, too.