I'm not sure anyone knows when this valve is working or not... as Jeepman noted, replacing it with a hose may seem just as good, at least for the front-seaters!
If it's flowing heat to the driver when the valve is closed and the flow is low, then I'm pretty sure it's flowing heat to the driver when the flow is higher as well... regardless of whether the valve is opening up to reduce restriction at high flow or not. If valve doesn't open, then only real difference is a little more heat than usual goes to back, a little less heat than usual goes to the front, and more pressure than usual is generated at the restrictor.
As I understand it, the whole point of this valve is to keep the rear heater core from being starved at low coolant flow (low RPM), so a valve that never opens at high RPM/Flow will work just as well for that... unless you are capable of discerning the likely small delta in front/rear core heat exchange if the valve fails to open. All I can say is it's a good thing the body of the Dorman is fused if it's not opening correctly! And, hey, maybe that is why it is fused! I can see it now... "hey, our valve keeps blowing apart at high RPM!!! Well, we'll fix that... we'll just fuse the ends together! Drats... that still didn't work, now the nipples are tearing out! Well, ok so put some reinforcement ribs on each end cap to keep the plastic from stress fracturing... hey that works great now! Wonder why the OEM vavle doesn't have such issues???"

Meanwhile, we're all really impressed at first... "wow, look how well built the Dorman part is... they fused the body together and added reinforcement ribs!!!" LOL!!!
FWIW, when I replaced the original leaking OEM valve, an interesting observation was that the restrictor plate "hole" was pretty much apparently worn open to the larger diameter of the seat/restristor-plate outer ring... resulting in a much larger hole and likely much less restriction than intended by design anyway, so these valves may long be pretty worthless well before they start leaking! And, nobody ever complained about a lack of heat coming from the rear vents... even if that was the case! Only got replaced when it began leaking... which I imagine to be the case for most folks.
As for front line technical support competence, I once asked KYB front-line technical support as to KYB's perspective for defining designations of "left" and "right" for the purposes of assuring correct placement of their struts on my vehicle as they intended. Just wanted to make sure we were both referencing it as viewed by the driver sitting in the driver seat while facing forward. Would have thought this sort of question would have been readily understood and easily answered by a company who designates product placement as left/right; however, after a couple of back and forth e-mails, my question had to be elevated to a higher level. To give credit where it's due, I did get a competent response once my question was elevated!