Good news ! I managed to figure out a workaround.
By setting “Allow upsample2x” to “On” I only need to change the “H.samplerate” to 857,95 or 858,25 for this problem to dissapear.
Moving the “H.samplerate” to a few full values below or above 858 meant my checkerboard was going all blurry on parts of the screen.
With this workaround the checkerboard in the 240p test suite looks juste fine !