What you are referring to is the range estimate, not the fuel gauge. The fuel gauge is the bars on the right side of the display and corresponds to actual fuel in the tank. The range estimate on the other hand is based on recent driving and is not always an accurate prediction. There was a software problem on earlier models that caused the range estimate to be way too high, like 900 miles, that was fixed in a software patch. I am not aware of the problem causing the estimated range to appear too low.
The most important thing is how many miles of HV driving do you actually get out of a tank. Unfortunately the car does not display HV miles anywhere so you have to track it yourself somehow, maybe keep a notepad handy and jot down the starting and ending mileage each time you use HV. Or use one of the trip odometers, reset it to 0 when you start driving HV, then note the number of trip miles when you switch back to EV. You just need to do this for one or two tankfuls to give you an idea what your actual HV range is, and also dividing HV miles by the number of gallons you filled with give you a fairly accurate mpg value, probably more accurate than the mpg numbers that appear in the display.