Spotlighting a post from reddit, this is consistent with what my guild has observed and what has been posted already in this forum, apparently without much notice:
"Wrath of Ragnaros is the notorious mechanic that you call your melee out/in for so they don’t get boop’d into the nether, its supposed to reduce the total threat of everyone caught in the radius by 25%.
The strategy was to always have a MT and an OT swap places in between “boops” so that the MT never drops threat, its horrendously bugged.
CURRENTLY: Wrath of Ragnaros wipes the threat of EVERYONE in the room by 100% irregardless of if you’re hit or not and to top it off, only 3 people are hit by it in order of of lowest to highest GUID (Globally Unique Identifier, aka when your created your character an ID is assigned to you) and the earlier character creation you have the higher on the list you are to be get punted.
The GUID bug was always afflicting Onyxia for several weeks, its since been fixed but now my fear is going into BWL onward the same type of bugs will be reappearing to drastically change the fights."
The present work-arround is just WAIT a few seconds before the melee go back in. This is what we have been doing and it works perfectly fine.
Not saying Blizz should ignore a problem if it is indeed a problem ( I don’t remember it working like this either) but who knows memories from 15 years ago are fuzzy in some ways, but the only thing I know for certain are the class mechanics and bugs there because muscle memory does not lie.