https://www.youtube.com/watch?v=R13tOou249M&feature=youtu.be
2:30 features royal decree activating.
It seems the ability doesn't activate at all beyond the sound effect and notification. You can see that brood chambers don't receive eggs, eggs don't hatch instantly, and the food counter continues to reduce during this time. You can also see there are not maximum ants, so that's not the issue.