I've only checked into this on a couple of occasions, but my impression is that my PHEV indicates a zero mile electric range when the state of charge falls to 20%. So if 20% is zero miles and 100% is 26 miles, then every 10% above 20% would constitute 3.25 miles of range (8 * 3.25 = 26). 50% would be 3 * 3.25 = 9.75 miles, which the range gauge would report as either 9 or 10 miles. That's kind of in the ballpark of what you're describing.
2018 Niro PHEV Gravity Blue