How to get appointments to automatically assign when importing as CSV?

When I import appointments via CSV, some of clients get their appointment automatically assigned to them, and some of the clients don't. In all cases, the client name, phone number, and email address are all exact matches. There are not multiple clients with the same email address, phone number, or name.

I've looked at this help section, and don't find any clue that applies to our situation: https://help.acuityscheduling.com/hc/en-us/articles/16676924729997-Client-accounts-in-Acuity-Scheduling#01FQHRV5E5YMQC1FGTHY92K7H5

Could it be possible that there are "hidden clients" or something that share the same email address and therefore are confusing Acuity?

How to get appointments to automatically assign when importing as CSV? And idea?