Wrong phone number associated to a name in Messages

When I type contactA's name for a new message on my Mac, it displays a phone number and converts the name to contactB. All old threads with contactA now show the wrong name too.

Possible root of the problem?: Last week I was updating contacts, and these two HAVE been on group message threads together. There was a group thread with about 8 people on it, and I was trying to make sure I had a contact record (i.e. name) for everyone. However, I think both of these contacts were already in there.

I've checked/tried:

1) Both of their contact records are correct.

2) Deleted both records and restarted my Mac.

3) My iPhone behaves correctly.

4) On my Mac, I have iMessages on, but have "Enable Messages in iCloud" off.