The Cloudron LDAP server is basically implemented on our own based on the ldapjs module.
In your case you would probably hit this function https://git.cloudron.io/cloudron/box/-/blob/master/src/userdirectory.js#L145 within which you can see the user attributes available. Those are basically hardcoded as we assume clients can also be configured accordingly so we didn't want to add extra complexity through further dynamic settings.
From https://social.technet.microsoft.com/wiki/contents/articles/52250.active-directory-user-principal-name.aspx looks like userPrincialName is similar to sAMAccountName but maybe more often used in AD rather than LDAP.
If this is typical for AD, I guess there is no harm done to add this attribute also in our directory server simply. Does anyone here on the forum have more experience with ActiveDirectory to contribute to this topic?