Mission Code Manager - How do I remove a MC from a train?

[davidbird]

I have used MCM (version 92.1) and allocated a MC to a train in initial session setup. However the train is showing with [no mission codes] - but the codes previously allocated are not available to add, so I presume they are still allocated to the train somewhere.
As there are [no mission codes] showing I cannot remove the MC added in error.
Is this a bug? Or am I missing something obvious?
Thanks.