You have a test port extension that is a cable? All the ones I have ever seen are only about 2 inches long and machined, not cable. If you do a 1 port at the instrument connector, then connect a load by a length of cable, you will see the usual peaks and valleys caused by the imperfections of the cable. The better the cable, the smaller they will be. If you are seeing 25 db of return loss at low frequencies from a load that you used to do the one port with, it sounds like the test port extension is bad. 25 dB at 3 GHz, maybe. But at low frequencies, I am like you, you shouldn't be seeing that. Interesting problem I will be waiting to see what you find out.