I think the issue here is your contact syncing service trying to override what you're trying to do.
They should be using one account/service like Google or iCloud or Yahoo and not all together as you get this round robin effect of one service overriding what you are trying to do.
To stop this you will need to stop syncing more than one account on your Mac.
To stop Yahoo sync:
How do I stop Yahoo Sync?
To stop Google sync see here.
To stop iCloud sync:
Choose Apple menu > System
Another issue is the phone can be updated from other devices as they are set to say iCloud but the iPhone is set to "Keep on Phone".
If the issue persists it is probably a bug, you can report it here.