I think you answered the question yourself: enabling the highlight and shadow module applies some transformations on the picture that, when pushed in extreme condition, render the background as black.
I tried with your image and if you switch the highlight and shadows module to use a bilateral filter, it behaves as you expect.
I don’t know the exact maths behind that module, but the color space it works on and the fact that at high exposure, your white area becomes clipped, is probably triggering some weird extremes.