The SharePoint Online People Picker returns users by First and Last Name for organizational user accounts and First/Last Name + email address for external accounts. For some tenants, such as Educational Institutions, there can be multiple users with the same First and Last Name, likewise there can be a user with two accounts having the same first/last name (e.g. student and employee). With the modern sharing experience, the inability to see an email address when using the SharePoint Online people picker has led to inappropriate sharing with users as those sharing files are unable to discriminate between the user accounts (e.g. documents shared with a student account by accident). Of course, there is the option, at the tenant-level, to enable search by email address or UPN only (and disable search by first/last name), however, this is not without its own issues in the modern UI.

This request to to all for additional details to be displayed in people picker (e.g. email address, affiliation, etc.)