I mean physically, how well-seated are the relay expansion modules plugged into the pass-through socket on the expansion dock?
Short of a circuit design problem or defective parts (I am not yet convinced of either, just trying to chime in to help), another area you can turn your attention to, to try to solve the problem, is to see if the stuff is physically making contact for all the signals.
If you have an intermittent connection, something like what you are seeing is possible.