The luminous intensities for the gamma angles 175 ° and 180 ° are 0lx for all levels. Therefore, one would expect the value 0lx in the middle of the ceiling.

Since we do not have a punctual luminaire here, the luminaire is divided into DIALux.
The rectangular luminous area is 1.2m x 1.2m and is divided into many small luminaires with the same LDC (light distribution curve).
Up to gamma = 174 ° one has with the LDC quite large luminous intensities.

This is enough that many of the small luminous partial areas emit light towards the center of the ceiling.
If the luminous area is reduced to 1.2 x 1.2m to 1cm x 1cm - it is practically punctual, it is black in the middle of the ceiling:

Alternative: You can also raise the room height to 14m with the same lighting position.
The effect of the luminaire division is then no longer effective and it is also black in the middle of the ceiling.
Here, a luminaire must therefore be considered as a flat object.