Yeah did only 4.6% of my damage last night on Helya mythic as opposed to 11% on first kill the week before (can't link prior week its a private log). And for the night on Helya it was down around 3%, although a couple of those didn't get to p2/3 where it really shines.
https://www.warcraftlogs.com/reports...-done&fight=23
I didn't notice if it double procced or not, but your theory seems consistent with my results above. big caveat very small sample size.