I recently had this problem after I charged the battery to full. My house is near a very large hill, and whenever I fully charge the battery and go down the hill the regen adds additional power back into the battery. It would always happen at the same intersection, right after being fully charged. I wonder why KIA doesn't do what the other companies do, and disable regen breaking when the battery is full.
I am not sure how it works, but I think the BMS notices this and freaks out by shutting down the car; probably to prevent the battery from overcharging. I got around this problem for a while by putting it in D mode when the battery was fully charged. I also learned later that whenever this was happening it was damaging my battery. So it is a good idea to take it to KIA and get it fixed right away, it is a safety issue.
They replaced the battery, but then I was unable to recharge the vehicle, I took it back to KIA recently, and they said it needed a new BMS.
I have 2 vehicles that allow full Regen at 100% indicated SOC
The 2015 Soul EV + and a 2023 F150 Lightning Pro SR
The Lightning has a larger buffer than the Soul EV so I assume it would be harder to get near actual pack capacity to stop Regen from working.
My 2015 Soul EV still does Regen even in D, so that seems weird that your Soul won't Regen when in D but only in B.
My Niro regular Hybrid technically still regens even when indicated battery meter is full, as this vehicle has a small 1.32kWh (battery) but it may just be using power dumped elsewhere (usually always have climate on....regening near indicated Full sometimes spins AC faster....It might hit overcharge protections if Climate control was off*)
My 2019 Bolt disabled Regen, if charge status is too high so 94-95% is upper limit of being able to retain Regen.
Loaner EV6 has Regen at 100% indicated State of Charge. (I suspect there is a large enough too end buffer)
Tesla+Toyota (2nd Gen) Rav4EV had normal mode and Extended Mode.
-When in Normal mode the Battery can show as 100% indicated State of Charge but if you switch to Extended Range mode the State of Charge would lower.
(Best practical example of having a top end battery buffer. I assumed Manufacturers which want to give the impression of more reliability with driving Regen characteristics and also flatter charging curve would prefer to have a larger top end buffer. The typical end user will just think the battery management software is better than other vehicles, when really the customer is just locked out of using most of the battery they paid for.)
*Technically a vehicle could be released where it always tries to prioritize X amount of driving range and then uses the rest of the battery capacity as "reserves"
Example would be a 200+kWh Silverado EV 4WT that instead of 450 mile of max range...was instead programmed to give 250 miles of range. (Battery would still charge the same, but regardless of weather conditions you would have roughly the same range. Basically the inverse of Power Reserve for using power outlet capacity (vehicle load) where you set a limit.)
Porsche have a rather large buffer, coupled with understating max driving range ....to where it seems they outperform EPA range and also flat charging curve when near 100% indicated capacity.