/place structures will not spawn their assigned mobs naturally

When spawning in an ocean monument using the new command /place structure there seems to be an issue with identifying and defining the bounding box for the new monument.  The structure spawns and follows the correct orientation.  It also spawns the three elder guardians; however, normal guardians do not natually generate in the world around the newly spawned monument.  

Same for any older structures, such as pillager outposts.

If this is intended behavior, I believe it should be changed.  Allowing the structures, such as nether fortresses, to spawn their respective mobs allows for much greater world customization.