Hello keyable,
The brightness setting is not applied during the orthomosaic export. If you need to modify the colors of the output orthomosaic, you can either use Color Matrix in the CIR calibration tab of the Raster Calculator dialog (for RGB imagery) - substitute the values in the main diagonal of the matrix according to the brightness improvement factor (so instead of 1,1,1 use 2,2,2, for example, for 200% brightness); alternatively you can modify the output channels in the Transform section of the Raster Calculator dialog for each channel and use Index Value option in the Raster Transform section of the Export Orthomosaic dialog.