This is normal, but it is not true that DL will only show DL operated flights. Be concerned if downstream flights are not shown. DL doesn't always show all flights prior to DL operated flights, but should show all after.

DL will show, and usually check you in for, even non partner flights that follow a DL operated flight. If they don't there is probably a problem. They do this because when an itinerary starts on DL they are responsible to get you to your final destination of anything goes wrong before handing you off to the next airline. On the return, they are not responsible till you actually arrive at your transfer point, so not seeing your earlier flights doesn't have any practical effect.