Same songs are getting re-synced every time

That's true, but to be clear for anyone reading: anything that was in the 'Grouping' tag referenced by a third party will be in both the 'Grouping' and 'Work Name' tag in iTunes. HOWEVER, updating from the third party appppp no longer updates the Grouping tag. It updates the Work Name tag. So as long as you aren't changing anything, you're good to go.

worked like a charm - many thanks

I'm having the same problem. The same batch of 30-36 songs always get re-synced whenever I sync my iPhone 6 (iOS 10.2) with iTunes 12.5.4.42. I tried removing all the songs from my iPhone via iTunes (unchecked "Sync Music", synced, and re-checked "Sync Music") and I tried checking the "Use work and movement" option for all of my songs, and the unchecking it, and then syncing, but neither solution worked. In fact, the amount of songs that re-synced increased from 30 to 36!

Not sure what else I'll do at this point. I think I'll eventually go on to remove my iTunes library and restore it again, and maybe remove the songs from my iPhone itself. I don't want to have to figure out the batch of 36 songs that refuse to sync normally... that'll require me to record my screen. I shouldn't have to do this, Apple! Please give us the option to generate a text change log with every sync.

About the group of songs that keep syncing: none of them have custom lyrics or any lyrics at all (the lyrics box is always unchecked because I never bother with song lyrics for any of my songs), and they're all encoded differently. Some of them in iTunes, other in LAME (LAME3.92 to 3.99). All of them are id3 2.3, and all of them have album art (different sizes).

Since I can't edit my post more than once, I'm replying to it.

So, the songs that are constantly re-syncing all have "Grouping" tags associated with them, which I added myself. Yet the "work and movement" trick didn't work for them, which is weird. I added info to the "Grouping" field in iTunes, not through any third-party program, so I don't know what the issue is.

I figured out the issue! It's the "Work" tag after all!

So the "work and movement" grouping tag was messing up the metadata of some of my songs. This included 31 of the songs that always re-synced--I had to choose the "Use work and movement" tag for them, and then I had to enter a backspace where the "Work" field was. They stopped syncing after that! All of the songs had filled "work" fields. I removed them.

For four other songs, I had to choose "Use work and movement" for them as well, but in the title field ("Work") there was a ??? instead of a title. I simply entered a backspace in the "Work" field after mass selecting all four, and they too stopped syncing!

The 36th song I fixed by creating a new version of the MP3--removed all properties and imported it into iTunes, and then manually added back the metadata. But that was before I figured out the easy solution. I spent a good two hours combing through my songs in foobar, trying to figure out which tag was causing issues. Turns out it was iTunes after all.

The songs don't need to be grouped, but I liked having them grouped. It's a shame that iTunes is forcing me to remove the grouping tags. 😢

This fixed it for me... So pleased. It has taken umpteen tries and a number of days before I stumbled upon this.

This just made the problem worse for me. Now the number of songs that get resynced went from 400 to 7,000