I have an iPhone 8 & iPhone 10 the 8 is on T-Mobile and the X on Verizon. Both are running iOS 11.4 both are signed into same iCloud account.

I puchased a ringtone from the iTunes app and tried to set it as a custom ring tone for a contact. So if I set the custom tone for a contact on the X then the 8 changes back to default, then if I go back to the 8 and change it back to the custom tone the X changes back to default.

I thought this might be a license issue I.e. the tone is only licensed to be used on 1 phone at the time so I purchased a second unrelated tone and tried to set the X to custom tone 1 and the 8 to custom tone 2.

However I get the same issue, even with 2 different tones, one of the 2 phones is always reset to default, no matter what