I've tested Mp3tag v3.28e and I think it works well.
No major issues as far as I can see, but here are some observations.
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.
When you enter a single value in the TVEPISODE field then any multiple occurrences get tidied up into a single T=50 PART_NUMBER tag.
Other than using DISCNUMBER field as previous workaround (prior to this fix), it doesn't really make much sense to try and enter a DISCNUMBER value for MKV anyway.
There's nothing much else that could be done with it, apart from maybe creating a non-standard unofficial DISCNUMBER Matroska tag, so I think the observed behaviour is alright.
Any MKV tags such as Director and Genre sitting at level T=30 (since they were created using an earlier version of Mp3tag), will remain that way unless actually edited with this new version of Mp3tag.
For example, an edit that changes Genre but leaves Director untouched will move the Genre tag to T=50, but leave the Director tag where it was, at T=30.
This means you'd have to do a dummy edit, changing the value of Director, saving it, then changing it back again, to get it to move to T=50.
Again, I think this is okay though.
It would probably create more problems than it resolves to try and automatically correct the target type value levels of unedited tags.
Mp3tag's TITLE field continues to produce TITLE T=30 tags (ie. at chapter level) for MKV.
There's an argument for setting it to level 50.
However, it's still possible to use the ALBUM field to produce TITLE T=50 tags for MKV, so I think that's fine for the time being.
Similar to the way that DISCNUMBER field entered values now disappear and pop up instead in the TVEPISODE field, Mp3tag's SUBTITLE field values still disappear when entered for MKV, continuing to pop up in SETSUBTITLE field instead.
There's an argument for reconsidering this behaviour now T=50 is the new default level for MKV.
But, it's just how it was working before, and thus, using the SETSUBTITLE field, it remains possible to create SUBTITLE T=50 tags for MKV, so it's not something that should hold up Mp3tag v3.28e going live I'd suggest.