For the NEST package, there is a statement to manually add the correct site-packages path to the PYTHONPATH.
Unfortunately, the current implementation only works if Python 3.8 is used as the path is hardcoded.
The statement itself is needed because only lib/python3.X/site-packages gets added to PYTHONPATH automatically by spack.
On some systems, the installation is done in lib64/python3.X/site-packages. Unlike in a virtual environment, lib64 is not a link to lib and therefore not part of PYTHONPATH.
In order to fix this, one can point CMake to always install to lib instead of lib64 (Source).
Issue has been found and fixed with the help of @terhorst.