The slim overflow on the reefer contains the return plumbing as well.
If you are experiencing the water in the overflow getting sucked out, that means that the return plumbing itself has a break in it, I think. Its similar to how a python water change works: You run, and it 'sucks' into the line. In this case, the return the pump is pumping water up from the sump, but its 'sucking' water from the overflow into the return.
Thats my guess. I have zero idea how the plumbing inside a Reefer series is set up or what fittings they use, but I'd start seeing if you can identify a spot where there might be a poor fit, broken seal, or something of that sort within the overflow itself.