Connectorly for Jira and WorkflowMax uses API calls to perform its tasks and needs an intuitive connection - the email address - to bind Jira Users to WorkflowMax Staff Members.
Jira Cloud developers, to be GDPR compliant, decided to remove the possibility to return the 'emailAddress' to the Admins even if the 'view' permission of the field is set to 'Only You and Admins'. This problem only affects the API layer, their web User Iinterface still works.
Currently, as a workaround, to see missing Jira Users in your Connection Dropdown List you'll need to ask your Jira Users to set their email-address 'view' permission to 'Anyone'. This needs to only be done once.
Also, please upvote the following ticket to the Jira Devs team for making this behaviour consistent between API and User Interface: Ability to retrieve Users' email-addresses (you'll find the 'vote' button on top-corner-right)