This is another mystery. Trying to get additional engine braking via manual downshift in Sport mode is an exercise in futility. Why then does programming force engine downshifting when the battery cannot absorb any more regen? The only reason I can think of is that the engine has variable valve timing and can improve actual engine braking. If that is the case, fuel use is virtually absent and the display change is just predicated on not being in EV mode.
Lots of reports of engine coming on when battery is saturated going downhill, but they never give enough details to suit me. I'd be interested to know if this behavior happens when cruise control is on and set to a specific speed and if the speed is maintained (a low set speed on a really steep hill would be the most interesting). Alternatively, does the engine come on when you are just "coasting" downhill with your foot off the throttle?
By the way, there is one situation I'm in almost daily where the instant mpg display is clearly screwy. I'm at the top of a short hill in EV (in my hybrid) and shift into neutral for a half mile coast to the next stop (if I leave it in gear, I cannot make it to the stop). During this period, no EV light is lit (can't be in EV as I'm not in gear". Yet before I come to a complete stop, the instant mpg is close to, but not maxed out. If the light is red, I put it back in gear, and regen immediately shows on the power meter, but EV light is still not on. As best I can tell audibly, the engine is off the whole time. I guess I could put on my OBD reader and see what engine rpms are doing to tell if it is on or off.
2018 Kia LX HEV Metal Stream with Advanced Tech