The game ended itself when Vergamot Body1 HP bar went down to 0 and I levelled up at the same time. Around 12:57 there was some weird bug on my screen.
Unable to replicate. We are aware of rare occurences of graphical issues during bossruns even after Update 65. Besides the proposals made by user Jooon above, you may try this experimental fix https://royals.ms/forum/threads/experimental-ht-crash-test-fix.161831/#post-951852 which helps some users.