"If you drop your phone in a river of molten lava, let it go, cause man...it's gone." Jack Handey
Well, that may be true. But, if you fail to sign out of iCloud before trading in your phone all the lava in the world is going to do you no good.
I also have this problem. It may or may not be connected to the above issue. In iTunes under "Managed Devices", my iPhone 6+ is listed as iPhone 5S. The association date is the date I activated my new iPhone 6+. So I'm pretty sure this is my iPhone 6+ misnamed. I also had ANOTHER iPhone 5S listed in Managed Devices. Its association date was the date I activated my iPhone 5S...about a year ago. I removed that phone from the list and it stay removed. I also removed my iPhone 6+ (named iPhone 5S) from the managed devices list and turned off iCloud on the 6+. It disappeared from Managed Devices. Then I turned iCloud back on, on the 6+. It reappeared in Managed Devices...as iPhone 5S! For some reason iTunes is stuck calling my 6+ a 5S. That may have something to do with the fact that I set up my 6+ from a backup of the 5S. What's odd, is that in all other ways my 6+ gets referred to as an iPhone 6+.
If this doesn't straighten itself out in 90 days I may just go looking for that volcano.