Falstead Darak. graphical bug/purple shape

Hello everyone!
I encountered a graphical bug. I’m not sure what the exact name of the issue is, but it happened.
On October 26th, I was playing in the new world with my second character. On the third try with Darak, I saw this purple shape on the arena.
The boss fight went normally, no other bugs occurred during the fight. After I left the location and returned (I defeated the boss on this try), the graphical issue was gone.
The boss was displayed and moved correctly during the first and second phases.

I can’t provide the save before the fight, but seeing all the bug reports in forum, I decided to write about it since I still have the screenshot saved.
I have no idea how to reproduce this bug.

Just mentioning for the record, I was switching between this world and the first world (to equip) with this character, where the story had already been completed.