Agreed. I was so glad I killed ashvane before the first 5% hp nerf because it was huge and after that many wipes we earnt the kill. To have it nerfed then walk in and 1 shot it like we did the following week would suck.
I think that after the boss dies once they should leave it alone unless there are obvious bugs etc.
Have some slowly increasing power creep like the azerite trait in uldir (I didn’t play WOD well enough to really remember the ring but it sounds similar in this regard).
That way there is no wondering if someone kills pre or post nerf. It is simply how soon it was killed because the “nerf” through power creep is constant.