Unified threads on Messages on iPad and iPhone but not Mac

I recently created a new user account on my Mac. Once I had set it up to use Messages in iCloud, there are two contacts whose messages are now separated into two separate threads (for their phone number and email address). This did not happen on the user account I used previously nor does it happen on the Messages app on my iPhone or my iPad. I have tried disabling Messages in iCloud on my Mac and on my other two devices and then re-enabling it but the situation remains: the threads are unified on my iOS devices but not on the Mac