Cockpit: cannot change MAC address of bridged network interface

After a VM install, I change the interface type to 'Bridge to LAN', with a source of 'bridge0', and default model ('Linux, perf'). This works fine if I don't modify the MAC address. If I boot the VM and run 'ip a', this shows that the interface name is 'enp1s0', as expected, and that the interface is UP, and has a new IP address from an external DHCP server.

However, if I attempt to change the MAC address, 'ip a' shows a new interface name of 'en7ps0', which is DOWN.

This is repeatable - going back to the original MAC address gives me a working enp1s0, while setting a different MAC address gives enp7s0, which is DOWN.

Any ideas? Is this a known bug, or am I doing something wrong? This is on RHEL 8.5, Cockpit 251.1.