Sorry I know I wasn't clear, I was abbreviating. I was trying to say I don't think the EPA can effectively mandate (intentional or not) via the design of their tests that a car can only have one starting mode, unchangeable by the driver. I.e. if the automaker allowed the driver to control whether the car starts in EV mode or HV mode, the EPA might choose to start a particular test run in HV mode when that would be less advantageous for the manufacturer in terms of rating. Thus effectively forcing the automaker to allow only EV as the startup mode.
A complicated thought to be sure, which is why I was saying I don't think it works that way
I should circle back to what I should have asked instead - what type of test scenario do you think might be negatively affected by having sticky HV mode? The way I think of the tests is they run separate tests for EV mode and HV mode. They start full charge and drive only in EV mode and measure miles driven vs. kWh used. Then repeat the EV test for different driving conditions, recharging to full prior to each test. Then for the HV test they start with depleted EV range (but full gas tank) and then measure miles driven vs. gallons of gasoline used. Repeated for different conditions, refilling the tank between tests. Sort of like the same tests we would do ourselves (if we feel ambitious) except more controlled and with more precise methods for measuring fuel and electric consumption.
If that's how the tests are done then HV stickiness wouldn't come into play because the tester is never pressing the HV button.