We've switched to using Azure AD authentication for ArcGIS Online. For guest users (not in organization), Azure AD creates a username (User principal name) that includes special characters (#'s), ex. somebody.somewhere.org#EXT#@acme365.onmicrosoft.com. This username cannot be use for the SAML ID when adding a member to ArcGIS Online. See image below.

Has anyone seen this problem before, and is there a solution?
Thanks,
Frank