Hello
I have 2 Macs all running the latest MacOS and an iPhone running the latest iOS. All three devices are using the same master iCloud contacts list and there are no other contacts accounts activated.
One one Mac I have an issue whereby some of iMessages are showing as phone numbers rather than resolving to the contact name.
logging out and in again to iCloud and rebooting has not resolved the issue. Anyone else seen this?
Not sure if this has been solved yet but i think i may have found a solution. It worked on my macbook pro at least, hope it does the same for you.
Go to Settings > ICloud > Check CONTACTS box. This synced my contacts to imessage and contact names started appearing on imessage on my mac. Hope this helps.