Probably all is well. Don't get hung up on the "range" statistic. That's just a calculation based on your recent history and is notoriously inaccurate. When it shut off on you, was there an error message? Were you at 1 mph or less? Could be you footpad sensor released on one side so it thought you were trying to get off. Or a motor stall detect, which also can happen at very low speed. Check your footpad sensors and tire pressure. If okay, ride it and see what happens.

I turn down a really steap hill every day about 30%. Has never shut off on me. I agree with everyone here. Sounds like a sensor issue. For reference