According to this Apple iCloud Drive support page:
I’m editing a document now. When will my changes upload to iCloud?
When you'll see your changes depends on the app you're using. Pages,
Numbers, and Keynote upload files to iCloud regularly to make sure you
don’t lose any changes.
Other apps upload your files to iCloud only
after you save them to the iCloud Drive folder. To see your saved
files on a Mac, go to Finder > iCloud Drive
. On your iPhone, iPad, or
iPod touch, go to the iCloud Drive app. On a PC with iCloud for
Windows, go to File Explorer > iCloud Drive
.
If you still don't see
the latest changes or you see an "in-progress change" icon or the
iCloud Drive icon in the notification area (system tray), try to save
a new document to the iCloud Drive folder. Then check the iCloud Drive
folder to see if the changes appear.
Found a kind of workaround to re-upload those related songs. Make sure you have a backup of those songs before trying in case something will go wrong, and make sure you have exported your custom created playlists to import them later if any of those songs are there.
You are going to remove those Matched or Purchased kind of files and will create new ones. These steps should save all the metadata like ratings, play counts and so on, but will have a new one date added to library.
iCloud should have to check those files again and will make those songs Matched or Uploaded once again, but this time everything should be playable and downloadable for offline use.
Make sure you have the iTunes Plus setting in Preferences > Import Settings.
Create the new playlist and put all songs you found broken in your previously created Smart Playlist, just in case it will not mess up later. Create a new Smart Playlist with todays Date Added, this will put all fixed files there that you will create later.
In the next step you have to Create AAC version in File > Convert. This will make a duplicate file but all the crucial metadata should be the same, file size may be larger or smaller though.
Now you need to completely remove broken songs under Song > Delete From Library.
This will give new Removed iCloud status to the newly created files. You will need to select all files in the new Smart Playlist with Date added today setting, right-click and select Add to iCloud Music Library.
You will have now Matched or Uploaded iCloud status in these files but everything should work properly now. The location of files eventually should be the same as removed ones, so you can have playable Exported playlists that you want to Import now.
Best Answer
I was concerned I'd have to save these documents to my local filesystem, delete them from iCloud, and then perhaps save them to iCloud over again, but it turned out that simply re-opening the "Waiting" documents, making a trivial change to them (such as deleting a character and then adding it in again), and then saving the file caused them to be synced successfully.