I was soloing ulduar25 for some achievements and I noticed that Mimiron got like 2x more hp than before. On first phase it's 12m second is 12m too and I didn't push the red button (the red button shouldn't add 2x more hp aswell and it's 15m hp now with hard mode). I couldn't do this fight because of this stealth buff and the 15 min enrage.
Screenshots:
First phase
Second phase
They used to have around 6m hp. Why would they buff Mimiron only?