Boss is invincible

Bug Report ID: #4174535165

Died to a t17 Ziggurat boss once, when I came back, it had no HP bar and was completely invincible to all attacks while still attacknig me.

If it helps - the boss summoned both mini-bosses at once. After both mini-bosses were defeatred and the boss lost some HP, it entered the invincible state, and since there were no mini-bosses left it stayed invincible.

Tried dying or leaving the game, but the boss remained in the invincible state.