Have you restarted Control:PC on 'B'?

There are many reasons why it can be, but don't be fooled by the local zone. A 'local' zone is only local on Control:Web. With Control:PC/:Mac, it's still a remote zone, it just happens to be hosted by your Control:PC on 'B'. Sounds like the zone has been lost as far as core on MC200 is concerned and is not re-establishing for whatever reason, hence not showing as a zone on anything.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).