Guest users with blank email address

I have an issue along the same lines.  When an external user is invited to a SharePoint online site they will successfully authenticate and be able to work in and out of the site for many months.  Then all of a sudden I will get a call that they no longer have access.  The user now receives a message that they can't be found in the SharePoint directory.   Correlation ID: 42ca299f-4029-a000-c897-f68b345e2098 and Issue Type: User not in directory .  When I check the permissions on the site for membership the user is listed.  When I go to the Admin Center - Users - Guest Users the user is listed but the email field is empty.  

My solution is to remove them from the regular membership page then go up and change the page to -0 and remove them from that page, then go to the admin center and remove them from the guest users.  I then wait at least 3 hours, most of the time I try and give it 24 hours, then go back to the site page and send a new invitation.

I would like to know what causes the email to be dropped from an external user?  When you share a page you have to put in an email address.

Robert can I assume that you used their external address to share the page to them and that they worked for a while?  But they are contacting you that they don't have access and you are now checking the guest users area and not seeing their email address field populated?