Office 365 Room delegate emails not being received

We have an Office 365 client that has two Room mailboxes – Boardroom and Training Room.

A delegate is assigned to each mailbox to approve/reject Room calendar bookings.

When the delegate clicks Accept or Decline the requesting user does not receive an email. The user only receives the "Tentative" email advising "your request was received and is pending approval."

Both the delegate and requesting user are internal users on the same 365 tenant, same domain.

Can someone please help?

Room mailbox delegate settings:

Sample email received by the delegate below.