I had the same problem, but with Appaloosa that I was growing out for Russ in 2013. I was alarmed when it didn't come out looking like its parent since I was responsible for returning seed back to him. I planted it again in 2014, this time in a raised bed with amended soil, not our heavy red clay. More pods successfully filled out with mature beans, but still, solid yellow, no white spots. Production was overall quite poor.
Giant Red Tarka also comes out differently for me. I did get some white spots, but they were not nearly so "giant" and lovely as the seed I had received. Twice I've grown it, once in the main garden, once in the raised bed. Same result as the Appaloosa.
There are some beans, primarily southwestern varieties, that I can scarcely get seed from if at all, even though the vines will grow and look nice and set pods. Some of the pods will look big, but contain only stunted beans. Other varieties grown side by side with them will produce just fine. I am less inclined to think that the soil's nutrient content or pH is at fault, but perhaps it has more to do with how clay holds onto water or maybe even our humid summer air. Pods that have come on with different timing when the weather was drier seem to have better luck. Those beans are just not well adapted to my climate, I think.
We have had some very cool and wet summers the past few years. It has made the whole garden a bit more of a challenge. I wish we could send a little of this rain over to the west coast. I still have beans to plant too. Hopefully we are coming to the end of this long rainy spell.
