When mirroring components in an assembly, if any of the components have constraints to origin planes, it creates a new work plane over top of the origin planes. When this happens I always go and change the constraint reference to the origins and delete the extra planes. I would be nice if Inventor could just recreate those constraints to the existing planes rather than making new ones. Maybe it could even recognize part faces and recreate constraints to those parts that aren't a part of the mirror.