I have encountered a "bad" copy of two lenses over the last 15 years but I did not expect this possibility to apply to the much less complex TCs. Now I am not so sure. I have both TC14 and TC20. I used them to look at 800 mm using the TC20 compared to stacked TCs. The reviews indicate the TC20 should a better choice. I took several RAW photos taken at 80 to 100 meters using a Z9 + 400 TC 2.8 on a Gitzo series 3 tripod. Examining the photos on a 27 inch UHD monitor at 200% there was certainly some softening, particularly in the corners. I could not see any difference between the TC20 and stacked TCs. Maybe my TC20 is not as good as some others.
Side note - I changed the camera and external TC without moving the lens and tripod. I could not see any difference in FOV. I suspect that 1.4 is just rounded off from the square root of 2 or the difference between 784 mm and 800 mm is pretty small.