The sensor reacts to ambient light conditions as well as direct sunlight. It is the intensity of light that activates the sensor. I have this happen to me occasionally: The main beam will come on in entering a darkened tunnel but will still be on even when I emerge into lightened conditions. This mainly happens when, though it's reasonably bright, there is not the correct intensity of light to activate the sensor to deactivate the main beam and hence your internal lights. However, if your internal lights are still on when the DRLs are on and not the main beam, then there is something wrong. I cannot quite see how this can happen. Have you checked that the internal lights work in tandem with the main beam? If they do, then all is well. Also, the main beam can be activated in very shady conditions even when there is bright sunlight.
What I don't know is whether the sensitivity of the light sensor can be adjusted.