Okay so things are stranger and my mind is hitting a wall.
I did the upgrade to 21.05.1 and it went super smooth (thanks Netgate!)
But!
I was still having the issue were traffic from a single IP address was not getting processed in 1:1 NAT.
Same as I saw in 2.4.5p1, literally any IP on the internet except the one from my SIP provider would be properly NAT'd and send through to the 3CX system.
Grasping at straws I was wondering if the state created by the 3CX registering with the provider was an issue since it contained the same IP and port info as the incoming connection? (Blue is my public address, Red is the SIP provider's)
Screen Shot 2021-10-12 at 8.29.46 PM.png
Screen Shot 2021-10-12 at 8.30.33 PM.png
Just for grins, I changed the trunk time at both ends to be IP based (no authentication) just to see if anything changed.
For reasons I cannot comprehend, it started working.
SAME source address, SAME destination address, but it's being properly NAT'd now.
I literally have no idea why that worked when the other way (registration based) didn't?
So I guess everything is okay now but I really really hate problems that don't make any sense and the resolution just feels like pushing off the inevitable when it breaks again.
Thank you for your help, and if there's anything I've posted above that catches your eye, please let me know, otherwise I will have to be half-satisfied that it works but half-unsatisfied because there's no logical reason for it to have not worked in the first place.