Here's what you should have linked :
https://www.warcraftlogs.com/reports...100&graph=true
Your logs don't really bear out that assertion - you realize that the raid will naturally take more damage towards the end of the fight because the adds in the stands keep increasing and throwing more bombs...right? It's a soft enrage mechanic.
You didn't use a mana potion, but it looks like you ran out of mana just as the fight ended (I tend to hit mine earlier so I don't ever have to worry about getting an "OOM" message).
Frankly most fights are designed to get more hectic / damaging as they go on (Kargath, Butcher, Tectus, and Mar'Gok solidly fall in this category), so the assertion than you stay above 50% mana for the beginning parts of the fight is completely beside the point.