Hi Dave,
Welcome to the Cisco Meraki Community!
Issues with websites being miscategorized have been addressed in MX Firmware version 13.3 or higher, which should be available in Dashboard, under Organization > Firmware upgrades.
If there isn't any MX firmware higher than version 13.3 available in your Dashboard, or the issue persists after the firmware upgrade, then I would recommend you contacting Cisco Meraki Support by going to Help > Get Help > Email Support and submitting an Email Case. Our Support Engineers will be able to assist you further with this issue.