The experience distribution is a bit buggy

As reported by CrashWolfy

The bug manifests as stopping at 1 exp remaining to levelup regardless of exp earned, then going to 0 exp remaining on next kill, then finally another kill leveling up without the usual fanfare or messaging, starting off the next level if applicable at perfectly 0% to next level again regardless of exp earned.
It's a little hard for me to reproduce to find the exact cause, but it's been happening on most levelups for me on my main character so it might just be at high levels (44 and 45?)
It also might be related to whatever is causing the steam overlay and hud to bug out after a second or further connection. Again, hard to tell because the only way to test would be to restart the game before leveling up.

Start by reproducing the reported leveling behavior around levels 44–45, including the experience remaining, level-up fanfare, and messaging after successive kills. Compare normal level-ups with cases involving a second or later connection and the Steam overlay or HUD issue. Done means experience distribution and level-up feedback remain correct and the next level does not begin at an incorrect percentage.