Bug no Uber Incarnation of Dread

I witnessed a possible bug that makes the boss immortal. The boss's mechanics require us to collect roses to make him vulnerable, however, it wasn't possible to collect the roses in my attempt to defeat him. I spent a lot of currency to buy the invitation fragments and lost everything because the boss was apparently bugged. Is there a mechanic I'm not aware of, or is it a bug?

Image to illustrate the bug:
https://imgur.com/a/KShEAHC