When booking a "Resource" mailbox and setting as "busy" (i.e. "accepted") the resource mail box request may be sent to the opposite side to a requested attendee before the booking side has completely book the request. This results in a "Tentative" booking vs an "Accepted" booking.
The Calendar agent will then sync that "Tentative" status back to the other opposite side. Both sides may show a "Tentative" booking when it should show the room as "Busy" or already Accepted by attendees.
The customer needs to have autoAccept configured.
If the Resource mailbox is not Switched, the Source Resource mailbox will need to autoAccept invitations externally.
One possible correction for this is as follows:
To resolve this problem, use the appropriate cmdlet listed below, depending on the results of the cmdlets provided in the Cause section.
Set-CalendarProcessing -identity alias -AutomateProcessing AutoUpdate
Set-CalendarProcessing -identity alias -ProcessExternalMeetingMessages $True