Hello Alterco,
Thank you for sharing the related data.
I see that you are using 16-bit input images and the resulting orthomosaic is also 16-bit, if exported to TIFF format. Whereas JPG export is downsampled to 8-bit output.
As for the data display in QGIS, it seems that it automatically stretches 16-bit range for the channels according to the image band histograms. Please open the imported TIFF raster properties in QGIS, then go to Style tab and modify the values for RGB bands in the Band Rendering section to 0 - 65535 range for all the bands. I think it should give you visually similar result to JPG file.