The mesh automatically generated during 3D interpolation of segmentations seems to have inverted normals. Tested on Ubuntu 20.04 MITK commit hash 1c4db16fd159fc20a3938e16cadcb2b0a8cdcb00 (current develop).
Found this bug when using vtkImplicitPolyDataDistance to calculate distances from points to the segmentation surface in MITK Diffusion, which means that it is probably only reproducible using MITK Diffusion (MitkCalculateDistanceToSegmentation cmdapp). Downloadable here: https://github.com/MIC-DKFZ/MITK-Diffusion/#Downloads
To reproduce, save AF_left_TDI and the respective surface. Call the command line app with these two items as input. The output is a distance map from each non-zero voxel in the AF_left_TDI image to the surface. If everything is fine, the values of the resulting distance map image inside of the surface should be <0 and outside >0. For the interpolation surface it is the other way round.