Just had this happen 2 man, with 125 decrees. Seem to be able to strong arm the wyrmlings in frozen circle solo okay with right frame. 2 man it wouldn't let us die, 0-8 wyrmlings after we killed them. He ended up leaving, then was giving me the option to revive Kaithe, but was useless. I dont think its acceptable to play 3 hours and have a stupid bug on boss fight. If you have a frame and decrees strong enough to handle the cold, let it be so.
Think I'll do the bonus undercrofts before boss moving forward.
I have a screenshot of option to revive Kaithe, that doesn't work.