White Balance Merge

I recently corrected the WB on a forest scene. I used 4 primary areas as ‘sunlit’ and each of those areas used the same width of merge outline. I then used the inverse to set the WB for the ‘shade’.
In general the results are good but there is one area where a hard-line is seen despite being well inside the shown merge area.
The question that I am asking myself is whether this is ‘natural’ (very hard shade), or whether this is a masking problem.
Ideas would be appreciated.

Parametric masking instead might be smoother…Background look like it is missing contrast and has some color artifacts…Blue or purple??? You could try now to go to your shadow WB adjustment and select custom form the dropdown no matter what it has chosen. Then just drop the chroma of the adjustment applied and use your eyes to see how it moves…this is the way I tweak the color calibration all the time ie select custom so it bring up the hue chroma version of controls and then just adjust the chroma to tweak the effect…Maybe also try color balance. A worked example with lighting and snow demonstrating how to tackle this is in the 2.6 blog post using color balance https://www.darktable.org/2018/12/darktable-26/