Currently, using HelloID Provisioning, HelloID has no way of knowing the mail address if you changed the mail address outside of HelloID.
This causes issues, as the mail address isn't provisioned to other target systems.
I would like to suggest to create a source system for AzureAD, and then correlate the users.
This way, the emailaddress can be imported from Azure AD, making a change on the person, causing an update on all target systems.