BryanC's answer is correct.
Windows Explorer won't see all tags in a file, and it will see different tags for different file formats. It sees mp3 tags because those are very standardised. FLAC is also quite standardised, using an Ogg Vorbis (Vorbis Comment Block) data chunk I believe, but still, something like ratings may not be saved in a way that Windows Explorer can see it. WAV and AIF are worse.
Make sure you have tag writing enabled, and that the MC fields you want to be written as tags are flagged to do so in MC's "Manage Library Fields" function. Then the only way to really test whether the tags are written to the file is to write them to a file in one copy of MC, and then import or update those files in another copy of MC (or another Library on the same PC) with writing Sidecar files turned off in the first MC installation so that the second installation doesn't just read the tags from the Sidecar file.
You can also use other tagging software to view tags in the files, if they use the same standards as MC. Many will, but not all.
Bottom line:
Tagging, even for audio files, is still a bit of an Art, rather than a Science. There are many "standards", and not all are very standard. For some file types there is no standard at all, but just a loose agreement on how files are to be tagged, usually by doing it the way the most popular programs did it in the past.