Meeting Request Do Not Automatically Accepted By On-Prem Rooms In Hybrid Exchange Structure

During a migration from an on-premises Exchange environment to Exchange

During a migration from an on-premises Exchange environment to Exchange Online, I found an issue where meeting requests for a room mailbox were not accepted automatically. This only happened with users who were migrated. After logging in onto a room mailbox, I found this error:

Appearantly mails sent from a Exchange Online account were seen as external mailboxes. The room mailbox was still on-prem.

To solve this issue, I had to allow processing of external mail senders. Basically, the PS cmdlet is something like this:
Set-CalendarProcessing MeetingRoomName -ProcessExternalMeetingMessages $True

For all mailboxes (rooms)
get-mailbox -RecipientTypeDetails RoomMailbox |Set-CalendarProcessing $emailaddress -ProcessExternalMeetingMessages $True