I have an MKV file that has cover art (Attachments) in it, but VLC doesn't show the cover in Tools->Media Information. Now it does show it for some files, but not for others. Using MKVInfo, the files that show the cover art have the Attachments above the first cluster, the ones that don't have it after the clusters. I don't see anything in the matroska docs about this type of restriction. There is a SeekHead at the start of the file that has records for the attachments record.
I can post an example file if that would help.
Dan