It affects every gamemode.
I play on pocket edition (not affected by the edition).
Before the kill:
After the kill:
When you level up, sometimes you may see that the xp bar is full on the level that you went. When you get some xp, it turns back to normal. It doesn’t affect the rewards that you get by leveling up.
Why this glitched xp bar even exists?
Note: The pictures are like that to avoid any complain from anyone.