I put some soy sauce and a generous amount of Frank's RedHot Extra Hot into my chicken stir fry but it was considerably less spicy than I'm used to.
Does soy sauce counteract heat / hot stuff?
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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.