Sorry you are quite right it should be fixed. But maybe it is matching to one release rather than another because of some other reason, the best thing would be for to increase the logging level to the highest setting then try and match, then send your support files that show me why it has picked one release rather than another.
The issue is not preferred media format that is working correctly...
The problem is the track and release scores were the same for the two releases. The release score for the digitial media was the same as for the CD release because although it got nothing for release format it got 20 instead of 10 for release date because it was the release with the earliest release out of the ones compared (even though the difference was the only a month). So we end up with two releases with identical scores, for consistency in this scenario they are just then sorted by the arbitary MusicBrainz Release Id.
If on the Remote Correct tab you set Preferred Release Date to No Preference then you'll get the result you want, but you probbably dont usually want to do this.
Not sure if there is much I can do to improve things, i.e if you want earliest release date and want cd release and you have two releases one that is better on one preference but not option which should be chosen