@Florian We also had this issue in Picard. It indeed seems to be that Windows only looks at the first portion of the file to read the tags, if it does not find them there it does not display tags. The solution I went for with Picard is to ensure that the metadata block is always written before the picture block. That might be something that could be handled by MP3Tag as well.
See https://tickets.metabrainz.org/browse/PICARD-1630 for details.
Just to be clear: This is a Windows bug, not a bug in MP3Tag or any other tagger. But it is possible to write the tags in a way that circumvents the issue.
Also just to emphasize how weird Windows can be I quote myself from the ticket:
Another interesting fun fact: Even if the Explorer is unable to display such metadata, it will cleanly write the data if you edit it in the details view. Explorer itself is afterwards still unable to read the data it just wrote, but other tools (e.g. Picard) will correctly display it. Windows is strange.