I have run several tests and I believe there is a bug with the dxf export option in TBC 5.0. The dxf file that is created is much larger than it should be. I'm using the dxf_2000 option, but "dxf_2018" option blows up the file size too.
I have attached examples.
DWG file is the base... I imported this into BC 4.12 - exported a dxf 2000 file - got a file size of 396 kb.
I imported this dwg into TBC 5.0 - export a dxf 2000 file - got a file size of 2,803 kb
Note: the file exported from TBC 5.0 cannot be imported into Terramodel, but the 4.12 export can.
Attached is the DWG file as well as the dxf files that were created from both versions.