itunes on PC autofills music on iphone 7 despite option settings

itunes keeps autofill music on my iphone 7. I have "manually manage music" checked device summary page in itunes & the "prevent ipods, iphones, &ipads from syncing automatically" option check in device preferences. However when I delete songs from the iphone itunes will start automatically coping / autofilling songs to the iphone. I'd like this to stop so that I can delete a bunch of songs then manually add new ones I want. Seems like itunes isn't looking at/obeying/caring about settings. It looks like itunes wants to fill in "missing" songs on the iphone i.e. the songs with gray dotted circles in the left hand column. I'd like it NOT to do this automagically. What else do I need to do to not have it do this? I could delete them on the iphones but I'd like to see which ones are missing before I do that. Is there a solution for this?