Seeing the same basic thing. Have a half-dozen songs where either the song title or the album title has parentheses, (and several have other weird punctuation marks or symbols, apostrophes and the like, ) and it won't let me UN-hide them! Frustrating esp. since there were about 50 others that I was able to unhide just FINE, but these... there's an UNHIDE button, and an UNHIDE ALL button, I click them, the button reacts to being clicked, but then nothing happens. In iTunes store, the song shows as being purchased, but there's no link in the drop-down menu to re-download it, which would be nice, since obviously it knows I've bought the song.
Between all the times iTunes has jacked up my library, renamed files, switched tags around, deleted files (they just suddenly disappeared! Where'd they go?) and the dismal failure that was my attempt at using iTunes Match for a day... I'm about finished with Apple, and ready to trade in ALL my Apple stuff for something that actually DOES just work. Left iTunes Tech Support a NastyGram to this same general effect. I have just about HAD it. Tired of having to recopy my library over and over again. Just glad I had the foresight to make multiple redundant backups of everything!
Let's see if Apple acknowledges the problem and offers to fix it.