Magic Circle gives +2 to all saves
Magic Circle versus alignment is granting +2 to ALL saves, regardless of what alignment is selected. Additionally, it stacks with Protection from alignment, and itself.
Scenario 1:
Cast Protection from Alignment(evil), Magic Circle against Alignment(evil).
An evil aligned spellcaster casts a spells that triggers a fortitude saving throw.
Expected:
The saving throw bonus should be +2.
Actual:
The saving throw bonus is +4.
Scenario 2:
Cast Protection from Alignment(good), Magic Circle against Alignment(evil).
A good aligned spellcaster casts a spells that triggers a fortitude saving throw.
Expected:
The saving throw bonus should be +2.
Actual:
The saving throw bonus is +4.
Scenario 3:
Cast Magic Circle against Alignment(good), Magic Circle against Alignment(evil).
A good aligned spellcaster casts a spells that triggers a fortitude saving throw.
Expected:
The saving throw bonus should be +2.
Actual:
The saving throw bonus is +4.
Speculation: It's behaving as the vanilla spell which states:
The bonus granted to saving throws stacks with the bonus granted by protection from alignment.
https://nwn.fandom.com/wiki/Magic_circl ... _alignment
Addendum:
Casting protection from alignment after magic circle will overwrite the extra +2.