How do you use the boss monster spell respawn
Based on Rules for 5th edition, page 5 (Rooms), this would only affect your visible cards.

http://www.brotherwisegames.com/wp-content/uploads/2017/05/Boss-Monster-5th-Edition-Rulebook.pdf


  B. Ability: The rules text of the card is active as
  long as it is visible and in play.


The Respawn calls for all of your rooms, but technically the text of the rooms that are not visible is not active, so it wouldn't matter if they had just been spawned -- they are not available to trigger the room ability text.