Yay! This was fixed in the release of macOS Catalina.
Replace the MAC address in the following command to whatever you want and it should work.
$ sudo ifconfig en0 ether 00:80:48:ba:d1:30
For almost a year the ability to change the MAC address on MacBooks has been broken or disabled.
I can't find any official statements from Apple clarifying but since this issue has persisted for so long I assume it is intentional. I really hope it's not.