I don't understand, doesn't all modding go through override folder? Meaning, if the file is not there, it was not modded?
I downloaded Near Infinity and opened chitin.key in BG2EE folder, searched for MXSPLSRC.2DA, the location is DATA/DEFAULT.BIF. Furthermore, isn't MXSPLSRC.2DA the spell table for sorcerer and not mage?
I tried adding XP via console to newly created fighter mage. It's interesting that the problem appears exactly when leveling up from fighter3/mage3 to fighter4/mage3. For example, if you level up from fighter1/mage1 to fighter7/mage7, there is no issue, the number of spells is correct. The problem is that leveling fighter from level 3 to 4 adds mage spells.