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...