That is just a visual bug probably caused by improved UI, I have the same. You don't have the experience needed to level up, next level is 300 and you have 251, just keep going.
This is correct. Blew my mind because I practically went through the entire Underdark and didn't level up. Or some other area. I may be getting my multi parties conflated. Either way that brings me to the next point.
Isn't the mustard coloured progress bar on the right your experience?
I did a few significant battles with one party in particular and it looked like they were perpetually stuck on the last 10th of that bar. That's what led me to my assumption that we now know is incorrect.