The EV range is at zero when I hit that downhill grade and does not increase at all going down the hill.
Only if the battery shows SOME mileage on it...like if it has 12.2 miles range showing on EV it will gain a tenth or two when slowing down or going downhill....but once it hits zero it won't show ANY increase, no matter what.
Oh okay that is something different. That's just how it works, once you get down to 0 miles then it will no longer display above 0 even if you do some regen.
First to split two things, there is SOC (state of charge) which is the percentage of battery charge, then there is EV range which is just an estimate and can be quirky, so you can't base too much on what the displayed EV range is doing. SOC percent unfortunately is not displayed anywhere on the screen other than the bars on the left, which I think you said don't move when you are going down Conejo grade? To see the precise SOC you have to use the Hondalink app, which does not update while driving, the app updates SOC only at certain events like starting charging or turning off the car. However they do make relatively inexpensive small bluetooth devices that plug into the OBD2 port under the dash, then there are free phone apps that will display information from the OBD2 device including SOC. Not expecting you to do all that unless you are really interested, so I will just give a general idea what you would probably be seeing if you did monitor it.
EV range of 0 generally will be when you are at about 10% SOC, plus or minus a few. And here's a quirk, if you are at about 2 miles EV range or below and shut off the car, when you turn the car back on it will be at 0 EV range, even if you immediately turned the car back on. Clearly the SOC didn't change two miles worth during that time, this just highlights the disconnect between estimated EV range and actual SOC. Think of it like a gasoline car, if you wanted to do precise observations of fuel consumption in different driving conditions you wouldn't go by the estimated range to empty feature if the car has one, instead you would connect equipment that precisely measures fuel flow and/or remaining fuel. Fortunately with EV's there already exists fairly precise data about SOC, even if it is a bit of a hassle to obtain it while driving.
Once you get down to 0 EV range then like I said it stays at 0 even though SOC might go up or down a couple of percent. The only time I have seen EV range go back up from 0 is when I either charge, or use HV charge while driving.
EV range will not go negative even if you go well below 10%. And even if you regen above 10% it will still be stuck at 0. I am pretty sure this is true even with a lot of regen like you experience on Conejo grade. My biggest downhill that I drive will gain about 2 or 3 miles of EV range if I am still above 0 when I reach the downhill, however if I am already at 0 EV range when I reach the downhill then it stays at 0 even if I do that same downhill. Now I just thought about it, what I should do is when I reach the bottom of that downhill I should pull over and shut off the car and check the app to see what the SOC is, which I am guessing will be about 15% or so, and then turn the car back on and see if EV range is now above 0, although I am pretty sure it will still be stuck at 0.
After the downhill I have an uphill the rest of the way home, including a steep incline at the very end, which uses up nearly all of the SOC that I gained on the downhill, which is why my SOC is back down to around 10% when I shut off the car. However I will do that experiment next time I am in that situation out of curiosity and check SOC at the end of the downhill. You can do the same thing next time on Conejo grade, when you reach the bottom if you can find a safe place to pull over and shut off the car then check what your SOC is and see if it is way above 10%. Unfortunately the app communicates with the car via the cellular network and Internet so if either you or the car don't have cell phone connection at that location you can't check the app and you will have to drive farther to where you have coverage, although it will probably burn off much of the SOC that you gained on the downhill by the time you drive somewhere that your are able to use the app. If that problem occurs then you would have to use the Bluetooth OBD2 to check SOC at the bottom of the grade.
Maybe that's more trouble than it's worth to find out what it is doing, but if you do wind up doing any experiments along those lines on Conejo grade be sure to report back your findings.
Although as a subplot to the story, as I mentioned there are limits to regen especially heavy regen when the battery is already warm. If you are able to monitor SOC during the downhill using Bluetooth OBD2 you might be able to observe this. But of course only safe if someone else is in the car to monitor the app, I think there are some apps that will record but that's a little more complicated. But for the basic test you just have to pull over and shut off the car then use the app to check SOC.