PSA: illusion spells break some followers (Remiel, Gore) and how to fix it
TL; DR casting illusion spells on some followers, breaks them. You either need to kill them or dismiss/re-recruit them to fix it.
Edit: current hypothesis is that the Mysticism - A Magic Overhaul (which is bundled into Nordic Souls) does something to illusion spells that causes this.
I am posting this with the hope that it helps someone else, because I spent hours last night searching for a solution and couldn't find one online.
I am about 90h into a modded game (Nordic Souls) and have a few followers, mainly using Remiel and Gore.
My Remiel, who was working fine (amazing follower btw, highly recommend), suddenly went completely off the rails: randomly walks off and disappears permanently. Reloading old saves, I can't trade equipment with her or instruct her to do anything.
Eventually I figured out that casting illusion spells like Command on some followers, including Remiel and Gore, breaks them spectacularly when the spell wears off. It randomly either makes them hostile to all things living, dismisses them in a non-recoverable way (they disappear from the world after walking off), or breaks their inventory.
I could fix Remiel by attacking her until she goes prone and then casting heal on her. I could fix Gore by dismissing and rehiring him. The fixes did not work vice versa.
Would be great if someone could confirm whether they can reproduce this. Might help narrow down where the problem is (other mod, general Skyrim issue or the follower mods themselves).