@Fervis
Here's the screenshot of Miraidon's error. Upon further testing, it only happens with moves that do direct damage; moves like Metal Sound and Mirror Coat work correctly.
And just to be clear: Koraidon's ability wasn't throwing back an error, it just wasn't triggering when it's sent out. I was fighting on Route 3, for what it's worth.
Thanks for the quick reply. If you need me to test further, please let me know.