First off, please make sure that the balance is running the latest and greatest 5.4.9 firmware just in case. You can download the firmware from the following link.
> Could the router still be holding onto the original (slower) ISP as the priority for some reason?
No it couldn’t. Configurations are set up in the way you see in the GUI of the Peplink Balance.
You might need to tweak the Outbound rules in the Balance based on your requirements if you haven’t tried it. Please see the following KB article to see if this helps or not.
http://www.peplink.com/knowledgebase/understanding-configuring-outbound-traffic-rules/
Please keep us posted. Thank you.