Yes, this is how the game works and has always worked. The hand that throws the cast references the weapon that’s in its hand. Since the spell cast animation set has a right/left/both animation, sometimes it uses the left, which most of the time carries an off-hand. This might be a surprise to you, but it is not to us.
This has largely been a non-issue over the years as most spammable spells do not have % Weapon damage, or only had low amounts. It’s also why you won’t see PRM on a two-hander with some big % weapon damage mod, for example.
Just like with the big armor and physical damage interaction controversy, this is how the game works, and always did. It is balanced around this behavior. It’s not going to change.