According to the manufacturer, this is intentional, not a defect. Soligor reportedly confirmed by email that the differing contact behavior is a normal feature of the set, even if they did not explain why it was designed that way.

So the short answer is: the tubes are supposed to be different.

That also fits your observation that the 20mm tube affects how the camera reads the Pentax A-contact, while the 12mm and 36mm do not behave the same way. Because extension tubes simply pass mechanical/electrical information through, a difference in that contact can change whether the body thinks the lens is in the "A" position, and stacking order can therefore matter.

If the behavior causes problems with your camera/lens combination, the practical advice from the community was to contact the manufacturer or retailer and consider returning the set if it doesn’t meet your needs.