@SeanB:
These move ~20ms or longer, shorter exposure times are made by forming only narrow gap rolling across the frame. So if the interference would be an issue, then it would made effects like
this one@Roi_hartmann:
In fact there are two points in the photo processing chain, that need exposure adjusting, what might be fooled: First is when the photo is taken to the negative and second is, when the negative is transferred onto the paper.
The second is usually automatically controlled in the processing machine, set to get equivalent brightness between all pictures in the batch, while compact color of totally black or totally white pictures might cause this second exposure control to act weird...
Or the problem with the sensor was all the way opposite: It was sensitive only for the yellow, so based on it's level it "think", then there is way more light then in reality and severely underexposed the film.
And if it was BW film, there is another problem with the sensitivity curve of these films: They are most sensitive i the blue end of the spectrum (where HPS does emit only very little amount of light, LPS obviously no at all), while not as much in the red part. This might make the situation even worse: Sensor set the camera for the amount of yellow, while the film is not sensitive there.
And other cause might be the wrong timing alignment between shutter and the flash lamp (if flash was always used inside): The automatic set the exposure for the flash, but it flashed at the time, when shutter was closed. So the picture was taken effectively without the flash, but rather fast shutter, yielding again severe underexposure.