@Mich **********
The way we know for sure is that if this was a requirement at a border bounce, it would be clearly stated on official website(s), just as all the real requirements are. It is stated nowhere. The only place this requirement has ever been stated, is in a bunch of anonymous posts... and some non-anonymous posts boldly claiming that an IO can always demand whatever they want and can always deny you entry, if they want, based on any imaginary requirement they can think of. I have entered on the DTV about 8 times by now. I have never been asked anything other than the printed visa.
I have also not seen any credible non-anonymous reports of that having ever happened