
First let me say I do not have very much knowledge of fits/seizures in dogs.
Now, let me say that from what you describe, this could be a state of intense stress/fear brought on by the ACP - it's a downright evil drug. It relaxes the muscles so that often, the dog cannot move, but remains fully aware; so any fear or stress about a situation remains in full effect, but the dog is unable to do anything about it. Your dog may not have been on a high enough dosage but there will have been some effect - pointless to prescribe it if there wasn't.
In that state, those emotions will get worse
because the dog cannot do enough to alleviate them - so right away, the ACP has made the underlying problem worse.
On top of that, ACP can induce a fear-state all by itself - so if your dog was unfortunate enough to have that heppen, then there's a triple-whammy going on: the original stress, the stress of being physically affected, and the stress caused by the drug. So it wouldn't susprise me at all if, once the dog was able to, it was in such a poor state of mind/emotion that it behaved as you described.
Possible fitting aside I would be making a complaint to the practice about the prescription of ACP as it is totally inappropriate for this problem (or indeed any, IMO, and smacks of a seriously out of date vet).
For the problem itself - if the regular vets aren't making any headway then it's time to ask for a referral.