Do iBGP speakers advertise their update source to neighbors?

I've got some iBGP speakers peering with one another using loopback interfaces.

Each router has introduced its own loopback IP to its local BGP RIB.

None of these routers are advertising that loopback IP to their iBGP neighbors. There's no route-map / filter issue, the rest of the local RIB goes fine. Just this one prefix doesn't escape.

Now, of course I understand that each neighbor device already has reachability to the peer's loopback interface... Without it BGP wouldn't come up. But I want this /32 prefix in BGP so it gets advertised to other places, either via iBGP route reflectors or to eBGP neighbors. With the relevant prefix sitting only in the owner's BGP RIB, it's not escaping the local AS.

What say you?

Is there some BGP rule which says we shouldn't advertise the prefix we're using to originate sessions with peers?

Final fun fact: I'm in PAN-OS town. I don't see this behavior on IOS.