Is Normalize emails enabled on your site, perchance?
Check if normalized email is unique. Normalized email removes all dots from the username and everything between + and @ symbols.
When I enable that setting, the error message when I use an email address with + character, I get is this:
An error occurred: One of the users you are sending this message to could not be found.
When I disable the setting, the PM is created and the staged user is created and gets the email.