When trying to create a room on Prosody instance, it throws an error item-not-found when we try to submit config form for non-existent room. AFAIR, it is allowed in a XEP-0045, in section "10.1.3 Creating a Reserved Room" with words:

If the room does not already exist, the service MUST return an initial room configuration form to the user.

however, Beagle fails with creation as Prosody returns item-not-found while Siskin handles this item-not-found by trying to join/create (using <presence/> and then reconfigure the room.