[Radek]

I see.
I am not an expert in BG2 logic, moreover, I am a fighter rather than a spellcaster so that I haven't noticed this behaviour. The level 50 rules do a simple thing: they extend the internal tables (XP for leveling up, saving throws, number of spell slots and so on) to level 50. I haven't found any .2da file that would say: "the spell power shall increase up to level 50". There are 2 possibilities:
1. The plain fact that the tables are extended to level 50 causes this unwanted side effect. In this case, ANY mod, which extends the tables, will cause the same.
2. The level 50 rules do not contain needed .2da file that would set the maximal spell power to a reasonable limit.
If the later is true and if you know what needs to be done, please let me (and others, too!) know. Such .2da file should be added to the level 50 rules at any rate.