I already posted this in the Steam forums for the greater visibility, but since I bought the game on GoG I might as well report it here too.

What happened?
While fighting Shidra of the Worldpillar I seemingly killed the two servants he was spawning before the animation was done, which resulted in the boss retreating in the background and healing without having any way to stop it.

How did it happen?
During the fight I kept applying burst stacks with the Burst Lantern until I reached 30, then I used the special attack while he had his hands joined and was spawning the assistants, or whatever they're called. It happened fast so I'm not 100% sure, but I think the damage burst registered while the boss was about to open his hands, so that the assistants had spawned but were still in his closed hands, and they got destroyed together with the boss taking the usual damage; they also dropped breath from the Craver trait, so I'm fairly sure they spawned before dying, instead of the burst damage simply preventing the spawn somehow.

After that, the boss retreated in the background and started healing as usual... but since the assistants were already dead I could not stop him from healing. I waited until he got back to half health and then I simply returned to the last Surrogate.

Media
I managed to replicate the problem on video.