The disappearing discnumbers in Matroska files

[ohrenkino]

I think that the described phenomenon is described in the thread that you linked:

@RobertM

When you enter DISCNUMBER for MKV with this new v3.28e, the field disappears from view in the tag panel, and the entered value pops up in the TVEPISODE field instead.
If you repeat the process (or if TVEPISODE already has a value), then the TVEPISODE field shows multiple occurrences separated by double backslashes (\), creating multiple T=50 PART_NUMBER tags in the MKV file.
I think this is okay though.

So, you would have to add a field for TVEPISODE.

@confucius

Does this mean I need to use some mapping?

The same thread also says that it is not possible to map anything for Matroska.