Some muxers like mkvmerge (or MakeMKV) add statistcs tags by default that enable MediaInfo to show the average bitrate of the tracks in the file to the user. If these tracks aren't there, MediaInfo simply doesn't know the numbers.
You can use mkvpropedit (a part of MKVToolNix) to add the track statistics tags to your file: mkvpropedit --add-track-statistics-tags <file>
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