Why does iTunes lose the Sync Music setting?

I have an iPhone 6s Plus. I don't use it to listen to music much, so I don't sync very often. It seems that every time I do, I first have to update iTunes. I've just updated to the current version: 12.7.2.58. I've added tracks to my iTunes library and now I want them on my phone. I have a fairly large collection so I can't sync the whole lot to the phone, so I have to be able to choose what I want.

I start iTunes and plug in my phone. After they've connected, I click the little phone button on the bar below the menu bar. I see my phone name, top left and two menus one of top of the other, on the left side: Settings and On My Device.

I click on Summary (in Settings) and scroll to the bottom of the page - I've got 'Manually manage music and videos' ticked on the Summary page, Options section. This setting is remembered every time, I think.

Next, I click on Music (in Settings). As with every other time, Sync Music is not checked and the page is greyed out. I check it and a pop-up opens which reads: Are you sure you want to remove existing music, films, TV programmes and books from this iPhone and sync with this iTunes library? Music, films, TV programmes and books synced to "my iPhone" from other iTunes libraries will be removed and items will be synced from this iTunes library. My only options are Remove and Sync or Cancel.

I don't want to remove existing stuff, I only want to add new. Am I doing something wrong?