i have also noticed this recently. let me explain a bit of the background
had a mac mini which i bought itunes match on and replaced some (not all) of my mpeg files with matched AAC files.
i then bought a macbook air but because of the small size of the SSD did not download the matched AAC files to it. at this point i pretty much stopped using my mini to manage my music and just used to MBA to add / stream files.
due to some amendments i wanted to make to some of the track info on an album i downloaded it. even though it was fully matched i was surprised to see some of the songs downloading as matched mpeg's. i was pretty certain they used to all be AAC files.
sure enough i went back to my mini and noticed that the album had originally downloaded all tracks as matched AAC files. i have checked the file thoroughly to ensure it isn't corrupt or anything and it plays perfectly. as a test i then deleted the AAC file and redownloaded to see what came up, it downloaded as an mpeg!
my questions to apple would be
why are these files now reverting back to mpeg's after being matched originally?
and (perhaps more importantly) HOW did they get this file to upload? is it even MY mpeg or someone else's that uploaded? as i have stated originally it matched so no uploading would ever have taken place.
this isn't a one off i might add. i thoroughly checked my entire library of 10,000 songs and have over 300 instances of this happening where i originally had the matched AAC file and am now left with an mpeg.
more strange itunes match behaviour. coupled with the constant 11111 error, the long 'sending information to apple' time and the clean / explicit debacle equals very shoddy service so far. and no word from apple on fixes or even an aknowledgement this is happening. poor.
@groovyd, I may have to retract what I wrote in that earlier post -- this is starting to sound like a pattern, not a one-off.
@l.bishop, I see where you're going with this. It almost sounds like Apple has smaller uploads of MP3's in its database (for reasons unknown) and it offering those as downloads versus the Matched AAC's. Normally I'd say that you uploaded them that way, but you're not the only one stating that you didn't.
I have a lot of songs in iCloud right now - I might have to do some downloading (when "Sending Information to Apple" improves) as you've both got me curious, as well as the OP.