Thanks Steve,
I do indeed see the same MAC on each port, and this is the upstream mvneta1. However, what's odd is I re-attempted this but using ports 3,4 instead of 1,2. 1,2 are currently active for everything else, so I wanted to avoid any more disruption!
Anyway.. this time I didn't get any MAC address flapping errors on the switch but if I pulled out cable A everything continued working. If I swapped them and pulled out B everything would drop. I left it for a good 10-15 minutes while I went and got a brew and it never moved over to the other interface, despite the port channel being up on the switch.
I'll ask a couple of our network engineers and see if they can figure it out.