This is strange EPMV wouldn't usually decrease with Premium in fact we tend to see the opposite. I is likely a coincidence.
Please check your earnings report to see if the EPMV drop happened the day where you first seen Premium earnings or was it just before.
If its the same day i would suggest reaching out to the support team to take a look at your account.
If its a day or 2 before Premium earnings are reported then it is likely this is not due to Premium.
Its important to think have you made any other updates or changes to the site around the date you seen the drop?
This is a great guide on Troubleshooting EPMV Changes