I tried to open the endings_segmentations files generated by TractSeg in volume render mode. Opening the file in 2D slices seems to work normally but upon switching to volume render mode the visualization vanishes.
I noticed that after combining begins and ends with
mrcalc *_b.nii.gz *_e.nii.gz -max *.mif the volume rendering succeeds.
After investigation with mrinfo I noticed that mrcalc implicitly converts the 16 bit signed integer file exported by TractSeg to a 32 bit float datatype (little endian). After converting all the files explicitely using mrconvert I can open all of them in volume render mode now.
As this does not seem to be wanted behaviour I thought I would report it here.