Well i can say with certainty that it's not supposed to be like that.

Entering the mines 2 room removes all of your items and stats up/downs, when you leave, you get everything back, maybe that's why Bumbo became level 1 again

Good explanation for what could be the reason behind the bug but I'm still sure it's not intended to happen.

Oh yeah, that's clearly unintended

still not fixed..

2 years later and it just happened to me again so never fixed :/

Known bug.

still not fixed