However, yes. After buying several Major XP Boost Elixirs, I've noticed they always end early.
An arguably inconsequential amount perhaps, but still. I'd say anywhere from 10-12 seconds, the little icon disappears, and I believe the effect does as well.
I was letting it slide, but this time around I was trying to squeeze as much use as I could out of the last minute and a half, and I had 30 seconds left to turn in the quest. I used the teleport hub, got back to my quest giver, and sure enough the timer disappeared right after it ticked down from 13 to 12 seconds left.
Had the timer not vanished prematurely, I would've most certainly made it on time.