So this a nice bug, or
Fun Fact, which is related to the XP bar.
74:

Before reaching 75, the XP bar goes full, then empty, without lvl up.

On the mouse-over you can see, that there is some missing XP for 75, so from this point the XP bar is not synchronised with the actual values, basically lies to you.
And there was the real lvl up:

I think this bug is connected to the general XP penalty, which can be seen below:
Quote
After experience is calculated, the following penalties will apply:
When you pass level 75 you get 1/2 the experience.
When you pass level 80 you get 1/3 of the experience.
When you pass level 85 you get 1/4 the experience.
When you pass level 90 you get 1/5 the experience.
Or maybe I am wrong, and XP penalty is not the reason. But anyway this is an interesting bug.