Henry
Sorry it has taken me a few days to look at this - so what I am checking here is that the DXF file from SCS900 has the 3D linework from the field survey and the Point Blocks that represent the points in it, and when it is imported to TBC the Points are Positive Elevations yet the lines are correct with negative coordinates - is that the issue that you are highlighting here?
I see that the SPJ file worked correctly so you are likely OK and that this was a Bug Report but non critical for you at this time - let me know if that is not the case.
The TBC Development Team have found the issue and it has been addressed in development builds at least, those changes will hopefully be made available in the next release / update.
Alan