Thanks for all the suggestions.
Try enabling General > Support Extra Long Filenames. The limit is for the full path, not just the filename.
I tried this, but it didn't work, unfortunately.
It's also possible that the files are actually imported but just not tagged to the correct album. Try searching for them by filename in the (all) Files view.
I confirm that the files are not imported: they do not appear in the 'Files' view when I search for them.
Have you tried to import a single problematic folder to see if it ignores the exact same files?
Yes, I had done this and it doesn't import the files.
Another thing I tried was to manually open one of the files not imported by playing it with MC (in Finder, right-click on the file > Open with > Media Center 30). Then in MC, I go to 'Playing now', right-click on the file, and select 'Import Into Library'. This does import the file.
Also, if I duplicate the file (in Finder, File > Duplicate), MC can find this duplicate. So it seems the issue is unrelated to the file name (the file duplicate has the same name as the original, followed by 'copy', i.e. 'foo.flac' becomes 'foo copy.flac').
Go over the tree on the left and click on data, then see if you see any files in the Data views there. if so, you can just change them all to Audio and be (mostly done with it.
I wasn't able to follow these instructions: I don't see any "data views". Could you please elaborate, or link to the relevant documentation? A Google search didn't help. Thanks.
I suspect that if you enable logging, do an import, and then capture that log it might have information that the developers can use to help identify the problem.
Thanks. A log is attached.