Get-spoexternalUser has Null value for InvitedBy

Hi all

Get-spoexternaluser does not always show data of InvitedBy. Sometimes I see the person who invited and sometimes it is null. Does anyone know why? 

Also the command Get-Spoexternaluser itself does not return anything when I use it with -Filter option or -site Url option to get a list of external users  in a site collection. The cmdlet seems very buggy. At tenant level it returns all external users but the url filters are not working properly. 

I can use Azure AD commands to get external user info, but they don't give any information regarding SP sites. 

Is there a c# equivalent for Get-SPOExternalUser cmdlet?

TIA