I was doing Mythic Eonar for the ?? time this week on a holy paladin. I happened to have the trinket Sigil of Algari Concordance equipped. I kept failing the encounter even though I knew how to do it and had done it several times that week. Sometimes I fail it once and then get the 99% buff she has for the second time, but something was killing me always in the second ship that I thought was related to the encounter.
Then I finally looked at the combat log and mending the cracks (https://www.wowhead.com/spell=452469/mending-the-cracks) was healing me for a red/negative number and killing me while healing me. Apparently what was happening is my trinket was procing on the second ship and I was taking fall damage or whatever because of the debuffs that happen at that point and the summon from the trinket was healing me for a negative number and killing me. I’m guessing the trinket interacts with the legacy raid buff in an odd way causing an integer overflow or something and makes it heal you for a negative number.
It didn’t one shot me but it rapidly killed me even through paladin bubble since I was being healed for a negative number and not damaged.