Unable to activate Demon Form during the Arbiter fight

Hey there- I've never written one of these before so forgive me if I'm doing this incorrectly, but here goes -

I'm a lvl 96 infernalist focusing on the demon form build - I keep the skill at 2 stacks (activations?) just incase I unexpectedly despawn and need to quickly respawn again (looking at you Viper Napuatzi from the Vaal City map, omfg)

I have been successful at fighting the Arbiter a number of times now with little to no issue - aside from, you know, fighting the Arbiter.
However, tonight - during his phase 2 - my health rapidly drains as though I am no longer able to sustain my health regeneration and I despawn from demon form in the middle of the fight. Now - this could be a skill issue on my end, but it does come out of nowhere, and my health shouldn't drain as fast as it does with the number of stacks I'm at + the recoup from damage I have, etc. I've played this character for over 500 hours ... I have a decent feel for sustaining things by this point.
The issue is that I am UNABLE to then re-activate my demon form. It says I still have it available (shows a 1 on the top left corner of the ability icon), but it's greyed out as though I can no longer toggle it? This has killed me 3 times now - and the first 2 times I thought surely I must be doing something wrong, but now I am certain this is a bug because I have no idea why it won't let me re-activate it again no matter what I do. (I just run around like a chicken with my head cut off trying to toggle it back on again while he chases me around the map with his giant-ass sword until my inevitable death because I'm quite literally helpless) (I also tried to pause the fight to troubleshoot and he DIDN'T pause haha - just another bug there I suppose?)

I hope this makes sense -- I hope this is the right place to report this --
and I hope this can be looked into!

Thanks very much!
(P.S. I have been loving this game so much, it's wonderful having a game to play with my husband! Thank you for all of the hard work you guys do - you rock!)