Get-MgAuditLogSignIn Doesn't Work on Guest Users

[Shoehorner]

The Get-MgAuditLogSignIn does not appear to return any sign in activity for guest users in an Azure AD tenant. It returns sign ins normally for AzureAD users.

[Shoehorner]

Closing because I found out the answer to this issue. I will update so others can get an answer though.

When querying for the sign in logs for Azure AD guests, you must use their Email value instead of their userPrincipalName that is shown in Azure AD. The email address (all lower case) is apparently their "true" UPN.