It can lock at the ogres, had all 3 done multiple times but it can stick on 2/3 and not progress. Actually not even sure you can wipe out of that so its end of run. Pretty much any of the steps that require something to complete before you can move on seem to have the potential to break and leave progress stuck.

I tried the event 6 times. 4 times I went to a locked mid-game session. 1 time the last boss soft locked, and 1 time I got soft locked during the portal phase. So far, what I observed was some weird interaction with the stunning emote modifier. I'm suspecting they are killing the shriekers before they can drop the shields of the nightmares.

Soft locked after 3rd encounter shriekers. Dunno why.