This problem arises when one has a downloaded ringtone which is from copyrighted material; I ran into this problem and after digging around in various places learned that the 30-second clip of "Dueling Banjos" I had created about ten years ago is regarded as being from the original soundtrack/album (which iTunes Music offers for sale). Copyrighted material, and although in the past it played just fine, it no longer does with iOS 11 and instead I kept getting something else, Apple's default. Although the "Dueling Banjos" 30-seond clip is still listed among my custom ringtones it can no longer be used as my default selection. I solved the problem by going to the Tones Store on my iPhone and browsing through until I found other versions which are purchasable ringtones and therefore usable. While I was looking around, I also discovered another favorite tune that I decided to use as my ringtone instead and purchased it, too. Problem solved.....
This is a somewhat different issue than the one of being able to transfer custom ringtones from one's computer into the "Tones" area, which can be done by drag-and-drop.