sure, might be different, but to me it seems unlikely that 2 things (setting his HP to 0 and the missing boundary at the stone wall) cause the same effect (him respawning at full HP).
especially, since a lot of people killed him since the update, and nobody reported him respawning “just” from 0 HP in phase 1.
both issues seem to happen in phase 1.
it might be likely, that the OP of this post killed him near that same spot, causing him to somehow be partially outside his boss arena, which set him back inside at his spawn point, causing the fight to start over.
plus OPs description:
I was standing at the arena entrance so maybe the game thought I just entered for some reason.
or maybe it was not him leaving the arena at this spot, but parts of the player model clipped through parts of that wall and what OP speculated happened.
yes thats speculation, but it seems the most likely explanation to me.
in any case, it might help to test this further, get video “evidence”, try to reproduce it and send the team the logfiles (just in case) see here: How to Properly Report Bugs!