JessieC Those tubers don't look 'cooked', they look like what I would expect to see from a young plant. I wouldn't expect to see the white feeder roots if the temps were too high. With severe heat, the mother tuber turns to mush, the ends of all the new tubers rot and there will be no white feeder roots. It looks like the clump is growing 4-5" below soil level? If so, it would take some severe heat to bake tubers that deep. I use a cheapo Walmart probe-type cooking thermometer to check my soil temps at tuber level.
It is possible heat is the main problem but now I am leaning more towards a fertilizer problem. Maybe excessive phosphorus similar to Ted's post above. Was this the bed that supplied the soil test you posted.
I guess if it was me I would try one more experiment. I would replant the clump in a hole (10" x 10" filled with a quality potting soil (maybe mix in 1/3 of original soil. I would dig the old plant, shake of the soil , cut stems above the third or fourth node, and plant immediately in the new soil at the same depth they were in the old soil. After watering in I would fertilize with Miracle Grow (or similar soluble 26-8-16). This will tell you if it is a fertilizer problem. As a control I would prune an existing plant the same as the replanted plant but do not fertilize. Comparing the new growth of the plants will reveal if it is a fertilizer problem.
I love dahlia mysteries. If the summer is long enough we might be able to do enough experimenting to kill off your entire bed of dahlias!🙂