Describe the bug
I faced some non determinism in #1892 which I traced back to spurious values of walll roughness.
I tried running UB sanitizer and valgrind, the latter finds one memory error when accessing the flatner, but theres nothing while it's built.
The ndflatner unit test also fails on my machine, both in release and debug builds.

Given the complexity of that class, and it only being used in one place, I just wrote alternative code.
@maxaehle can you investigate the issue?

Desktop (please complete the following information):

• ubuntu 22 + gcc + mpich (standard packages)