On build 0.C 6838, by activating the treamors then immediately exiting out and saving the event glitches where the amigara horrors won’t spawn once you load in. No obsession, no spawning, nothing.
By using debug commands I spawned in one horror and the obsession & normal outcome began.
Exited out and tried again, same result. No spawning.
I haven’t made any attempts to replicate this, so it might just be a single-version bug.