1. Save e.g. a matched 2-dics release, which will correctly apply discnumbers
2. Remove the discnumber and the totaldiscnumber in another application
3. Re-import the release in Picard, which will automatically load the matched release

At this point, Picard says the release is correctly tagged (displaying the 'v' next to each track) and in the info-window you can clearly see the metadata-fields for the discnumbers. However, the tracks are most definitely not correctly tagged... they are missing their discnumbers.

I have not tried whether this is only the case for discnumbers, or for other fields as well.