Figured this out the hard way. If you create a custom BFD profile in Active / Active HA mode, the BFD profile names need to be unique to each device. If you try to configure a BFD profile with the same name on both devices, BFD will not come up.
If you do try to configure a BFD profile with the same BFD profile name and you make and commit changes afterwards, the second PA will complain that the BFD profile does not exist when you try to commit changes on the second PA. In short, the second PA will have the BFD profile removed from the config. Again, this only happens if you try to use the same BFD profile name on both PAs.
This has been address in PAN-OS 7.1.7, which is now available for download. Please review the release notes for PAN-65738
You will find on https://www.paloaltonetworks.com/documentation/71/pan-os/pan-os/networking/bfd-overview the following line, "Active/passive HA peers synchronize BFD configurations and sessions; active/active HA peers do not."
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