It could be very difficult, neurological, but without an exam or a brain scan (CT or MRI), it would be impossible to say it would point this way in view of the normal physical results. My initial proposal would be to discuss getting a CT head with the vet.
Any possibility of intoxication? Although you expect distorted bloods, any jaundices such yellowing in the eyes, maybe repetitive liver function. It remind me of champ, Joe Biden's dog who died recently.