Hey thanks for replying!
It was during level up from lvl2 to lvl3 where there were multiple choices I could make on different screens.
I can't remember the exact wording, but on one screen I had to choose a new spell from multiple that were available and on the other screen, I had the option to forget and then learn another spell.
I chose spell "x" (don't remember the name and currently don't have access to the PC) on the first screen - and that spell was available after the level up was done. But on the second screen, the game let me learn a new spell without forgetting another which was issue number 1 that I didn't understand. But also I don't understand where the choice of spells comes from. I thought maybe on the second screen I have to learn the spell that I chose on the first screen so that I can use it in battle, but just for fun, I chose to learn a completely different spell (again, without selecting a spell to forget). But after the level up was done, I didn't have that spell available anywhere...
Thanks, this sounds pretty much how it is in the game.
I think my problem is with understanding the swapping at level up. As I was (trying) to describe in my other comment, during level up I learned a new spell "x". Then I was trying to swap in spell "y" without "forgetting" another spell first, because the game let me do this. I thought that I gained a new spell slot so that's why I didnt have to forget another spell, but after level up, spell "y" wasn't available.