Almost every one complained they can’t add a particular conference room to a meeting request in Outlook. They said the meeting room disappears from their meeting request. Strange, huh! When I checked it myself, sure the resource is added and after a second it did disappeared or removed automatically.
Without boring details of analysis, I found the conference room calendar settings is messed up (or someone messing things around). To test if this happened to you, do this:
1. Connect to Exchange Server (or Exchange Online) by PowerShell
2. Type this cmdlet to see if you get an error: Get-MailboxCalendarConfiguration –Identity <Conference Room Display Name or email address>
If you get this error, that is our/your problem.
WARNING: The object “…” has been corrupted, and it’s in an inconsistent state. The following validation errors happened:
WARNING: The end time that you entered occurs before the start time. The start time must occur before the end time.
so as per the warning says working hours are setup wrong. Correct it by using this below cmdlet. I specified the working hours for the room is morning 8am to evening 6pm. Change the timings to your needs.
Set-MailboxCalendarConfiguration -Identity <Conference Room Display Name or email address> -WorkingHoursStartTime 08:00:00 -WorkingHoursEndTime 18:00:00
That’s all folks. No more disappearing of the resource. If this fixed your issue, leave me a reply below. 
Anand, the Architect 