Let me share what I have found with this - it has been a nightmare thanks to Apple's sloppy implementation of the new "Work" field.
The constant sync'ing is indeed because of the "Work" field being populated, For mp3's, the Work field in iTunes is stored in the "TIT1" frame, aka CONTENTGROUP. If you are using CONTENTGROUP in a 3rd party tagger to store mood & style keywords and have enjoyed seeing them import into iTunes' Grouping field for referencing in smartplaylists, those days are over, that data is now in "Work". And Work is nothing but problems and causes constant resyncing issues, etc., and however long it takes Apple to fix this is irrelevant, you probably want Grouping to work again like it used to. iTunes now read/writes Grouping to a new mp3 id3 frame called "GRP1". So if you want to retain all your hard-earned mood & style keywords that used to be in Grouping, you need to find a 3rd party tagger that supports both the TIT1 (CONTENTGROUP) frame and the new GRP1 (iTunes Grouping) frame. You can then copy CONTENTGROUP to GRP1 before getting rid of CONTENTGROUP which is now useless for anything but what it was designed for, the Classical "Work". Fortunately a couple of taggers have started supporting both tags... Mp3tag calls it "GROUPING". And thanks to Mp3tag's recent update, my iTunes library and syncs are functional again. Here are the steps:
1) use 3rd party tagger to copy "CONTENTGROUP" to "GROUPING", and delete CONTENTGROUP tag.
2) go into iTunes, select all files, Get Info, check "use work and movement", clear out Work field. Uncheck "use work and movement", update. This is the initial first step needed to get all that pesky CONTENTGROUP stuff out of Work in iTunes' library and stop the madness. Should not have to ever repeat this step unless you ever import songs into iTunes with CONTENTGROUP populated in the future.
3) re-import all affected songs into iTunes's library. Sometimes iTunes is flakey about reimporting into existing fields and ignores your attempt, there are iTunes "refresh library from file" scripts out there to ensure all fields are refreshed from the file tags, use them. Grouping should now be populated and Work empty. Constant resyncing should cease, and all references to Grouping in Smartplaylists should work again like they used to.
As long as any songs are imported into iTunes with CONTENTGROUP, aka TIT1 frame, aka "Work" populated, the problem will return. As long as songs are imported into iTunes with GROUPING, aka GRP1 frame populated, iTunes Grouping will work, and won't constantly resync.