Private Relay Doesn't Play Nice with pf Firewall

As the title says - I have been having issues for quite a while where the system tells me that iCloud+ Private Relay is unavailable due to "incompatible" software (of course, without any indication of which software is to blame).

I have come to find via trial and error. It was completely by accident when turning off pf using Murus to confirm something unrelated to Private Relay...and I just happened to see the notification banner that Private Relay was available once pf was off.

Now I'm not sure why Private Relay doesn't like pf to be on, nor if there is a specific rule or pf function to blame (or if it just doesn't like that process to be running). And it is a bit disconcerting that Apple has set it up that way, given that pf is their own built-in firewall...but there it is.

AFW doesn't seem to be an issue, BTW.