Ideally the brightness wouldn’t depend on refresh rate, but it takes a minimum amount of time to shift out data to the panel for a row, and depending on the color depth, sometimes the data takes longer to shift out than the pixels need to be on. In that case, the panels can’t get to full brightness as they’re spending some amount of time with the LEDs off and waiting for new data per frame. Increase the refresh rate and that amount of off time increases, so the brightness decreases.