In the picture above can be seen that the interpolation within the quad elements is not symmetric.
The problem is even worse when using “use log scale when mapping data to colors”.

Is there already a way to render a symmetric color distribution?

yes this is a know artifact due to underlying OpenGL triangulation to render quads.

You may try to keep only one half of your dataset, then apply triangulate filter followed by the reflect filter. It will make the triangulation explicit and you’ll be able to make them symmetric:

I can only use the triangulate filter when using the extract surface filter before. Why is that? My geometry is only a 2d problem.

After applying the triangulate filter, my data is already rendered symmetric. There is no need to keep only half of the data set and reflect that half. So is this is a reasonable approach?