First of all, I don't even know if I should post this, since I have mods installed, but the only mod I have that affects bosses is the "Automatic Health Adjust" because I play solo: https://steamcommunity.com/sharedfiles/filedetails/?id=764204839&searchtext=health+adjust
When she is around 2600hp she becomes immune to any hit, after a while she seems to become vulnerable again but not for too long, Is this a feature or a bug with the game/mod?
Nobody had this kind of problem with that mod, so I don't know if it's a game issue or mod related.
Here's a video, you can see it clearly when she gets enraged:
Steps to Reproduce
-Enable lag compensation -Fight Dragonfly
This issue has not been confirmed by a developer yet.