MosaicPro changes negative values in the input data to 1 for the output data during mosaic process,if feathering option is selected.
If the seam feathering option is not selected, the negative values are retained in the output data.
This is a known problem and has been reported as (IM-20809) has already been reported. There is a workaround however. If the input data is already accurately orthocorrected, then use the "Compute Active Area" option while adding images to MosaicPro. This will avoid the need to use the feathering option and allow the user to maintain the negative values.