The Resampling function (Raster -> Geometric operations -> Resampling) is not working properly on 8-bit images. Particularly bright (values close to 255) and dark areas (values close to 0) in the individual bands of the RGB image are getting set to no-data in the resampled image. Downscaling (increasing the spatial resolution from 2 to 1 m) of individual bands works fine. But upscaling (decreasing the spatial resolution from 2 to 4 m) has issues with all interpolation methods.
Have you tried “nearest neighbour” that forces the use of only real pixel velues present in the input product?
Also, pixel-size is noot the same as “resolution” as you surely know.
Yes, I also tried nearest neighbor (just as bilinar and bicubic). All methods provide the same erroneous result.
By spatial resolution I refer to the pixel size in meter. In the resampling tool it is called “pixel resolution”.