[Neus]

The game will attempt to use your best casting tool always rather than your main hand/sub hand weapon if both are applicable, namely for instances where one weapon has spelledge or you have a racial effect that can let the weapon be used as a casting tool. So, this isn't really a bug, but more of the game trying to be convenient (so you don't have to toggle ambi a bunch).

I'm not really sure how this would be changed without introducing that sort of inconvenience.