Blank/Black geotiff files during split exports

During the last export for which I chose the split export option (unticked the single file option) I got the archive with the data all nicely split and georeferenced, but with some geotiff files inside, 5.87 MB each, that look stark black during preview and which contain no useful data (for instance from the files ortho-1-13_export.tif, ortho-4-11_export.tif, ortho-7-9_export.tif, ortho-9-8_export.tif, ortho-10-7_export.tif, ortho-12-6_export.tif, ortho-15-4_export.tif, ortho-18-2_export.tif and ortho-23-0_export.tif). Any idea why this happens? They do not belong there and should not be included in the downloaded archived export.