Boss cannot be killed and does nothing

Bug Report ID: #1237604143

After I've frozen the boss twice and lowered its HP, it started phasing but seems to have bugged out.

I cannot damage the boss, nor does it do anything. If I leave the game and rejoin, I get placed in the boss fight while it is still bugged.

See GIF below.
https://imgur.com/a/XPWzkr5

Edit because image block does not work with GIFs it seems