In iPhone Messages, the contact that I have for my cousin shows up as a phone number, but on all of my Macs that same contact shows up with his name in Messages. If I go enter Contacts (or Cardhop) on my iPhone and search for his name, I find the card with his name and phone number associated with it. The same is true in Contacts (or Cardhop) on my Macs.
I’ve quit all open apps on my iPhone, powered it down, and I’ve even done a force restart in the hopes that would resolve the issues.
Updated: I just disabled and deleted all of my contacts in iCloud, and then enabled them, and that resolved the issue. Yay, me!
New update: the above fix was very short lived as once all of the contacts finished syncing his name disappeared from Messages, again.
More updates: I just noticed that on the Lock Screen that his name appears when I receive texts from him, but his name still doesn’t show in Messages on the iPhone.