Saved both tCW files using TC v21 as stl, and a mixed result, both opened fine in MeshLab (free) and Cura, with no apparent deviation from the TCW file. But they would not open in NetFabb (Free version) until I had exploded once to get rid of part tree, and saved as ASCII stl.
No idea why NetFabb was being a bit fussy, Anyway, I did not see any unwanted groves in the part, I would doubt part tree has anything to do with it, but as a test, make a copy of the file (using save-as), in the copy, explode once to get rid of part tree, and save as stl, try binary and ascii, (click set-up button in the save dialog).
Can you post an image of what the problem groove looks like.
** Edit ** Are you doing any scaling in flashforge, I just noticed that your drawing is in inches, the part in my cura (mm) was the wrong size, it seemed to scale down to mm just fine in cura though, without any extra grooves. ** End Edit **