After level 20 you gain 1 Base attack bonus every 2 levels. It's set no matter what sort of class you've got.
At this stage your 22 paladin, 4 cot will have the same BAB as a 20 ranger, 6 wizard. BAB progression only counts up to level 20. After that it's set to 1 every odd level. (21, 23, 25 etc) Just like saves are set to +1 every even level, regardless of your class's save progression.
Not a bug
