If you manually create a meeting for a contact, this contact receives an e-mail with a meeting invitation.
However, this e-mail is also sent if the customer has not given any consent to receiving e-mails and also is not subscribed to any communication channels. To make things even worse, this also happens from within a sandbox, which should not send any e-mails to contacts other than those listed as HubSpot users in that account.
As this is happening in a system which is running with GDPR restrictions enabled, I think this is a huge bug.
Best regards
Stefan