Really is hard to say why then.
I know in the rooms before it, there are vampire fledgling priests that cast dispel magic.
I also know that, once, I saw after a very, very heavy lag shortly after I'd applied all my buffs to my character, they were all immediately dispelled, though I've only seen that happen once this year, and before the recent fixes to what was causing the massive lag a month or two ago.
Regardless the actual spell is working as intended during my testing. How or why that spell didn't apply correctly to just you / was removed from you is another thing entirely.
The average weather damage is about 5 and 6 in that area, so the area of effect type damage reducing it to 2s and 3s is as expected if you had the 50% immunity up initially. The Xeg-yi attacking you seems to be around the time of you bleeding out often (the log isn't far enough back to see if you were hit to bleeding out and stabilised first) so it sounds like, from those damage ticks it was applying correctly when you first entered, and that you may've lost the immunity after being beaten down past -10 HP and the game decided to set you back to a bleeding out status, though your wards would've been dispelled because of it, making you more susceptible to the negative energy damage running rampant on both creatures and weather.
But that's just my assumption, I've no way to tell for sure. Though in your picture, at the very top is a message indicating you were hit to bleeding out right then, consequent negative damage after that would likely have dealt full damage as there was a potential you were dispelled at that point due to bleeding out, or at some point earlier.
A far larger combat log set of data to determine when or why it stopped working is required.