I've noticed every once in awhile I kill a boss, eagerly expecting a level up, and then... nothing. Sometimes in multiplayer, sometimes in single player. So far I've only been able to replicate it with de rol le.
My game mode and difficulty

Starting experience
Experience before entering DRL portal (attained one level from dragon)
I'm intentionally only hitting DRL in this spot, away from his head.
The purple text is a lie!
Stat screen showing no increase to total exp

On to Vol Opt, I guess?
This spot is where I believe discrepancy occurs, because you can take this opportunity to shoot De Rol Le in the face. Sometimes aiming is more of an art than a science and I'm sure I've not hit the essential part that would count as tagging for experience. That said, I'm not sure why tagging would be necessary as the games I've played in have all been exp share mode, and one person mode always has exp share turned on. I've successfully gotten zero experience from DRL three times in a row now by only attacking his body segments and not his head. I suspect the other bosses have similar mechanics where, if you do not hit them "properly", you won't get exp from the fight, whether you did 100% of the damage to the boss or not.

My game mode and difficulty

Starting experience

Experience before entering DRL portal (attained one level from dragon)

I'm intentionally only hitting DRL in this spot, away from his head.

The purple text is a lie!

Stat screen showing no increase to total exp

On to Vol Opt, I guess?

This spot is where I believe discrepancy occurs, because you can take this opportunity to shoot De Rol Le in the face. Sometimes aiming is more of an art than a science and I'm sure I've not hit the essential part that would count as tagging for experience. That said, I'm not sure why tagging would be necessary as the games I've played in have all been exp share mode, and one person mode always has exp share turned on. I've successfully gotten zero experience from DRL three times in a row now by only attacking his body segments and not his head. I suspect the other bosses have similar mechanics where, if you do not hit them "properly", you won't get exp from the fight, whether you did 100% of the damage to the boss or not.