Per my research, expect external users, if users do not have Exchange Online license, the Email column will show blank.
Here is a workaround for you to show Email address for uses.
1.Go to the Azure portal -> Uses -> All users -> Select the user -> Properties -> Contact information -> Fill in the Email column.
2.Then go to user information list of the site collection (https://tenant.sharepoint.com/sites/sitecollectionname/_layouts/15/people.aspx?MembershipGroupId=0) -> Select the user -> Actions -> Delete Users from Site Collection.
3.Re-grant the user with propriate permission for site collection.
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