As per Microsoft Tech Community:
Feature update: Email sending behavior for Groups in Outlook
We have recently fixed the email sending behavior to a group, where senders had complained about receiving the emails they send to a group, back in their personal inbox. With this fix, senders will no longer receive the emails they send to a group, back in their personal inbox. The sender can receive email sent to group in their Inbox, they have to login to their mailbox using OWA, and then Settings->Mail->Groups->"Send me a copy of email I send to a group"
The primary complaints we heard were about reading the same message users sent to a group multiple times - in their sent messages, and their inbox.
We believe sending an email to a person or to a group should be consistent, and this circling back of an email from a group was just leading to confusion, unnecessary triage, and inconvenience for a lot of our users.