Hi! Tactical situation: After leveling up, they give 5 experience points, which can be spent on increasing characteristics. BUT I DON'T UNDERSTAND HOW TO CONFIRM THE SELECTION. That is, the game does not confirm this choice itself, there is no Confirm button in window, as in the skill window. Moreover, after a quick load-save, these points disappear without a trace. What the hell? How to deal with it?

I had the same issue. The window (Hotkey T) is draggable. I had to drag the window up so that you can see the confirmation button.

After some level ups I realized that the distributed experience points were not saved and I had the starting stats.
The experience points are not saved.

Thankfully there is a character editor with that you can add the lost experience points. So you don't have to start the game all over.

Divine Divinity - Save Editor v0.1.0b created by iZakaroN
https://www.moddb.com/games/divine-divinity/downloads/save-editor-v010b-created-by-izakaron