To put in historical perspective:
It seems to me that any problems with PIN preferring current Diners Club MC USA "not working" at some restaurants are mild compared to the problems we (who have had the DC USA long before it became an MC) had with being restricted to the Diners Club network. Keep in mind that it had nothing to do with the Discover network back then, and had way more limited acceptance at restaurants and other merchants than anything I can think of today in the US except perhaps Bitcoin. You had to look for restaurants with a DC logo on the door, and if they didn't have it (but had logos for other networks), chances are they didn't accept it.
And acceptance in many other categories was fleeting. I remember having to use Dillard's as a department store (not Sears, not Macys, not JCPenneys, etc) because it was about the only department store near me that accepted DC!
Plus I run into restaurants that don't accept Amex still these days in big cities in the US. Only a fraction, but annoying when I'm trying to earn a signup bonus on an Amex card and want to use SKyMiles Dining restaurants and it turns out one of my favorites in that program in a given area doesn't give accept Amex or doesn't give dining program miles for Amex.
So that's why I'm not surprised that my DC might not work everywhere, or that a given card may not work at every restaurant.