When you do the dataverse search for an account, the search shows results based on different fields. In our case the "Regarding" field from Appointments is one of the fields the search claims to be showing results from, but the search is not able to show anything related to this field. The regarding field is the only place linked to account when you create/track a meeting and is therefore the natural place you would expect to find appointments linked to different accounts.

In Power Apps for the field information "Regarding of Appointment" you find: the following:

Display Name: Regarding

Name: regardingobjectid

Searchable: Yes

According to what you see here you should think the field should be searchable. However, when you check the field properties on the Solution Layer is says: "issearchable: false"

When setting up "Quick Find All Appointments" and choosing the fields to "Find by", Regarding is one of the fields you may select. Even stranger that you may select fields to find by that is not searchable.

My suggestion is to change the field properties of Regarding on Appointment to "issearchable: true"