The reason for this most likely due to the configuration for the Room account (resource/room mailbox) in Office 365/Exchange.
The following PowerShell command defines that any private flag of the appointment is not removed on the Room account which means that private flagged appointments subject should no longer be displayed on the Room Manager once the command is executed.
Set-CalendarProcessing room1@domain.com -RemovePrivateProperty $false
Note! Replace "room1@domain.com" with the equivalent for your Room account in the above command.