I have an iPhone 7 Plus and have had this problem for several years. I have some shared albums with lots of photos from family members but I don't want to import those photos into my main library. On my Mac, my main library photos do not include these shared photos, they are only in their respective shared albums. Once I sync my phone then many (not all) of these shared photos show up in my main library on my phone even though I did not import them into my library on my computer. How do I fix this?