I set the control for manual mode, wide opened f stop, cranked up shutter speed for way above 1/250 (1/2000~5000) and triggered the shutter and strobe.
It resulted in over-exposure. It appeared that when I adjusted the shutter speed higher, exposure compensation was automatically pulled down to minus side and that the actual shutter speed is much slower. I noticed that the shutter speed was reset to 1/50 or so after firing the shutter which appeared to be the speed actually used.
Model is D850.