Anonymous participant 245 that's not universal. With the Non-Immigrant O-A if you enter on the last day of validity you get a full year stamp. People use this to get 2 years of of them. Elite used work the same way, you could get 6 years out of the 5 year visa. But they changed this around COVID time and now they only stamp you up to the validity, not the full year.
6 month METV people can stamp in on the last day for a final stay and get almost 9 months with an extension.
We don't know how DTV will work in this regard. My guess though, they will only stamp to to expiry. The legislation establishing it doesn't say regarding entry but it does explicitly say an extension can't go beyond the end date of the visa.