If we are talking about the -3 (why -4?) that we can see on the screenshot it is not a bug.
The -X level only happens when your units had enough experience for a promotion, but died in a battle.
Let's say I have 10 recruits, I can upgrade them all into a tier 2 but I don't do it, and I lose 3 in a battle against looter, it will display -3 levels for my recruits.
It only happens when you have a full stack of units ready for promotion, because if you don't, the game will kill units that did not have enough experience for a promotion, therefore you will never lose a level.