I just bought a Samsung Galaxy A15 and was trying to set an individual ringtone for my dad, but it wouldn't save for the life of me. I finally figured out why and wanted to let people know in case they have the same issue. I actually had the same phone number in my contact list twice. Once under Dad and once under ICE (In Case of Emergency contact). Both had separate ringtones. Once I set them both for the same ringtone, I was able to change it to the one I wanted. This way, I know when my Dad is calling. (I had Googled for almost 2 hours and called my provider to no avail. None had answers.)
So, if you can't get a ringtone to stick, check to make sure that phone number isn't in your contact list more than once with different ringtones. Hope this helps!
When you add a contact to a group, emergency contacts or favorites, the contact is not duplicated, it is just indicated as such. If you actually did enter the contact twice and it's a true duplicate, only one will be able to have a custom ringtone. FYI, it's not necessary to enter the contact twice to make it an emergency contact. All you have to do is indicate it as such.
