Thanks for the answer and for moving the thread to the right place!
Here's what I found out. I had one playlist that behaved the way I described. If I had the playlist selected to sync it would copy ALL of the songs onto my device, ticked or no. (All of the songs were unticked.) If I set the playlist to NOT sync then it would remove all of the unticked songs from my device.
I had another playlist that did not behave this way. It would only sync songs that were ticked (at which point it doesn't matter if the playlist is set to sync or not).
Then I noticed: The playlist that was syncing unticked songs had the setting Display using view: Track Details. The other one was Albums and Tracks. I'm not sure why this would make a difference but it does.
I don't know if this is a bug or a feature but I'm glad to have the functionality back.