I am not sure if this is happening with other decor pieces as I only have the rainbow one, but it is staying on top of the horse layer - it will not go to the "background" layer. I unfortunately do not have a screenshot of it, but it's exactly what it sounds like. Click the foreground button, click the move to background, it stays in front.

This is no bug. When you exit the decor editor, it will be behind the horse.

I recently thought this was a bug too but you have to exit the decor editor and reload the horses page, it will in fact be behind the horse.