Meetings are being redirected (not emails, only meetings)

Hello guys, I have a weird case that requires the community expertise.

Issue: When you send an invite to the Supervisors' group, the Backoffice group also receives it.This only happens with invites, not with e-mails.

Scenario

Exchange: On-premises
Group1: Supervisors
Group2: Backoffice

My first though was that the Backoffice group was member of Supervisors, but it wasn`t. It only had users.

Then, just in case, I checked if there were any delegation. No delegation.

Maybe some user had an inbox rule to forward/redirect it, but none had (checked with "Get-InboxRule -Mailbox <UPN> | Select-Object -Property Name,Description,RedirectTo,ForwardTo,ForwardAsAttachmentTo")

Also checked if this wasn't the case of iOS 10.3.3 forwarding the invites (https://support.microsoft.com/en-gb/help/4014990/meeting-invitations-are-forwarded-by-an-attendee-for-an-existing-meeti), but the users that have iOS are using 13.5.1.

This only occurs with invites. When an e-mail is sent to Supervisors, Backoffice doesn`t receives it. But when an invite is sent, both receives.

Any idea of what may be causing the issue? Any suggestions?

Thanks in advanced.

Alexander S.