thanks a lot for the report, I've managed to track it down. Spawning new enemies from the Mothman's ability wasn't inputing the moth's saves properly. Worse than that it was creating a null ref and softlocking the game.
There does seem to be an inconvenient workaround: alt+f4 and reload as you said, then try to kill the mothman before they can spawn anything?
I don't know if I'm going to get around to patching the fix in right away, because mothman should be very rare and there is some type of workaround? But we'll see.
Also that's a lot of mutations wow