Tested a cleric PC with the air and fate domains earlier, along with GSF Divination. On the spell changes list, it says that the AB bonus goes up to +25 with GSF Divination. This bonus shows up on the character sheet, but when in combat it only goes up to +20.
NWN has a +20 attack bonus cap when it comes to spells in particular (not including spell-like attacks), which means GSF true strike won't allow you to exceed that number in combat even though it will show up on the character sheet.
This spell change was reverted as far as I know. I don't think there is a bug here other than documentation.
Didn't EE introduce ini-file settings that can increase the cap on the NWN hard-limits like this one?