Just get stuck in the boss room and can't progress.
I don't know if it's a coincidence or not, but both were round 30.
The dramatic music for no respawns played when I would expect the portal, then no portal.
The boss was the shrieker.
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Just had this same thing happen to me, but at wave 20. Melted the shrieker boss real quick, then no portal was there to let us out. Suppose that’s one way for The Witness to win — don’t let us back out of the ship. 😉