It sounds like the files have been imported, but just aren't visible in the default Views.
Customise the Audio > Files View.
Leave it as a Panes View. Add [Media Type] and [Media Sub Type] to the [Location] and [[File Type] fields already included in the Categories to show.
Uncheck the "Use parent scheme rules for file display" box, and check that the "Set Rule for File Display" contains no rules.
Click OK, select "Save As New View".
You will now have a new View called "Files Copy" which shows all files in the Library.
Search (search field in top right corner of View) for your base music directory, for example "D:\Music" or whatever you use. If you have these music tracks under more than one base music (top level) directory, you could search one at a time to find "lost" files.
If the [Media Type] in the top pane lists anything other than "Audio", click on the other value(s) and check if your lost files are in there. Sorting the detail section of the View by Filename might make them easy to find.
Using the above View I was quickly able to find a whole bunch of Image files (Cover Art) under my base music folder that had been imported into my test Library in error, due to bad Auto Import settings. I was then able to quickly remove them from the Library. You should be able to find those files and quickly change them all to a [Media Type] of "Audio", if that is the problem.
If the above doesn't address the issue, you need to look carefully at the definitions of the Views you are using to try to find these files. The definition must be excluding the files somehow.
On a separate issue, all those files in your image have a suffix of " (1)", which implies that you have copied the original files into the same folder they came from at some stage, and if using Windows Explorer Windows has added the suffix, or if using the MC RM&CF function MC has added the suffix.
Could it be that all these "lost" files are actually duplicates, or you made duplicates and then moved them to another directory, or something? Looks like you have some more cleanup, or perhaps file renaming, to do!
Picard will optionally move files if you use it to fix tags. I think it may even have an option to copy files to a new structured location. That could be how these " (1)" files were created. Check the Picard settings very carefully. It can do a lot of changes very quickly. Or Picard could have just been changing the [Album] tag so that the "lost" files were just listed under a different Album name. It does that sort of thing a lot if you let it. i.e. If you do bulk lookups for Albums and accept whatever it finds without checking the detail.