Molten boss bugs
I can confirm that I’ve also had parties that I have been in sometimes spawn directly back into the boss fight, but it seems random to me. Sometimes it happens but usually not.
Playing as an ele for fractals I’ve also had the mist form being bugged as you describe it. Usually it starts recharging without being activated but I have had it go off successfully if I wait a few seconds before activating it. It is rare when it does work though. I have not had enough time to test it though to see if it’s random or reliable after waiting X amount of seconds.
Another bug which may not be limited to just this fractal but on level 33 the mistlock instability of “Outflanked” applies to all (or nearly all) of the bosses attacks even if you are facing them; creating basically a one-shot kill for every attack. I had this boss battle again on level 34, with a different group, and it was noticeably easier since I was not getting one-shotted from every attack.