If the GeoTIFF reader is asked to read a low resolution area of the source image, and overviews are disabled, it will also end up skipping subsampling, even if it was not disabled.
This happens here:
That return statements does not allow reaching, and calling, decimationOnReadingControl which would setup the subsampling control. It is also to be noticed that decimationOnReadingControl should be subject to the decimation policy hint:
However the above method does not even receive it.