Does soy sauce counteract hot stuff?
Soy sauce is not known to chemically influence capsaicin or capsaicin absorption. The effect is likely subjective and has to do with something like spicy heat feeling less objectionable in a higher umami dish.

If you used more oil or fattier meat this time, this might be the real cause of the perceivedly milder taste - fats strongly influence how capsaicin is perceived (they do not neutralize it!) because they dissolve and redistribute it throughout the dish.