My experience: Encountered the boss, it got unkillable, read about it, understood i should avoid damaging him while he was transforming as this was a known bug, went back to the boss, managed to clear it while avoiding the bug. (on 1st try) So, i can't really share or imagine your frustration, but i can understand it.
I am sorry to hear that this isn't fixed yet, as i was VERY happy to read that it was in the patch notes, making the game "gold" as far as i'm concerned, because this is / was to me the only really gamebreaking flaw.
I am currently leveling my second character, this makes me curious, i have to see now if the issue is still there! (i believe you but that patch note makes it intriguing obviously! Just feel like testing!)
Anyway, if not fixed, just chill and show some patience, everybody knows about that bug, it is indeed THE number one bug of the game and i'm quite sure that the devs will fix it as soon as humanly possible!
Aside from this, i think that the game is already amazing! Calm down man! :)