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Configuring High Availability: https://live.paloaltonetworks.com/docs/DOC-2926
After enabling HA, the interfaces on the firewall will switch from using the interface MAC address to a virtual MAC address. In my case, the Palo Alto updated the MAC address to connected devices, except for the loopback interfaces. I had to clear the arp table of my internet edge routers to update the MAC of the loopbacks (I'm terminating GlobalProtect to the loopback interfaces).
More information regarding the MAC address change can be found here: https://live.paloaltonetworks.com/docs/DOC-4144