I believe you are renaming your files in Windows using the window file explorer tool. If so, the behavior you see is expected. Here's what happens:
1. Some long time ago, you imported your files.
2. Then you added the Composition tag to some of your songs. MC stores this in it's database and associates it with that file.
3. Time passes.
4. Now you rename the files or folders using a Windows tool.
5. MC runs auto import and it sees:
A. Some of your files disappeared! So it removes them from the database. Including the composition tag.
B. There are also some NEW files with new names it has never seen before. So MC imports them and reads the tags from inside the files.
6. Now you look at your library and you see mostly the same stuff. But everything you renamed has actually been deleted and reimported by MC and thus has lost any custom tags that were not written to the actual files.
There are several things you can do it make this situation better:
1. Instead of renaming files and folders in Windows, do it inside MC using the Rename, Move, and Copy files tool. When you do this, MC is aware of the file name and folder name changes. So all of the fields in the database stay the same. There's no delete and import like before. It will just work correctly and retain all information.
2. Turn on the option to write tags back to files when they change.
Tools > Options > General > Importing & Tagging > update tags when file info changes
Note that this will NOT write any existing tags to the files. You have to tell MC to write to the file with Library Tools > update tags (from library) . Or change any value on the file and it will update all tags.
This should help keep more of your metadata inside your files. But for certain files like WAV for example, it won't be able to write "custom" or unusual tags like Composition. This is why I advocate for using RM&C instead of moving files externally with windows file explorer.
I hope this makes sense.
Brian.