Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
RecipientFilter is modifiable.
Ref: https://msdn.microsoft.com/en-us/library/dd264648(v=exchsrvcs.149).aspx
Your current setting only takes UserMailbox, you'd need to allow MailContact as well to include Contacts as DDL members.
Replace: (RecipientTypeDetails -eq 'UserMailbox')
With: ((RecipientTypeDetails -eq 'UserMailbox') -or (RecipientTypeDetails -eq 'MailContact'))
Let us know how it goes.