It seems a lot of people have had issues with new Jabsco joker valves and even though I could see some light showing through the slits at the end it did seem to keep the waste from coming back through. These are the ones used on the electric head with the macerator pump and are much smaller than the ones that have the manual pump.
This is an aged old problem with joker valves to be good the need to be flexible but they need to stay closed with pressure against them. They never do because the design is poor and the quality questionable.