I think you try to save in 8bits a 16/32 bits LINEAR image. So all low levels are removed, all your signal !
It is normal you get this.
You must "histogramTransform" your image (stretch the levels), then save your image
To do a quick transform, you apply a STF then drag the blue triangle of STF in the bottom bar of histogramTransform. Your image will be stretched like the STF
Cheers