Is the range shown in the "guess-o-meter" accurate to actual driving? - If you are fully charged, set your trip-counter, and compare it to the guess-o-meter range - a difference would point to a software issue. If actual and guess'd range are spot-on, then a hardware issue.
Since you have already tried several times through BMW Dealers, do a search for "BMW Customer Service", and contact them directly with this - certainly not normal at all, unless you are driving the car like you are running the Indy 500 - if you drive it like it's a race-car, the guess-o-meter will show that, and drop your range accordingly. You can easily knock 30% or more off of your range by "spirited" driving, just like doing so in an ICE car will reduce gas mileage significantly.
My car made a 50 mile trip from the Auto Auction lot to the Dealership, before I picked it up. When I started driving it, the guess-o-meter range increased over 20 miles - likely because the Dealership driver "drove it like he stole it" on the way back from the auction lot and that was what the guess-o-meter was calculating range from.
2015 i3 Rex, Capparis White, Tera World, Technology & Driving Assistant, Parking Assistant, Harman Kardon Audio System, 20 inch wheels, EVoInnovate EVSE