This being said, it should be clear that a wizard 1/cleric 19 having access to all cleric and wizard spells can't be the intention.
Well fortunately it doesn't work that way.
1 Cleric / 11 Sorc can only cast level 1 cleric spells.
The game is combining total spell levels for arcane or divine levels, in fact NWN1&2 already did this - 'Your arcane / divine spellcasting raises every level / every other level / every level beyond first'.
Seems to be working as intended. If you think it isn't send a support email.