Like your reader last weekl I acquired (on a lease) a new Hyundai Tuscon Hybrid in January 2025. I have done 6500 miles (varied driving) and have kept a spreadsheet of my MPG and brim the tank each time I fill up. What my calculations show is that the real cumulative MPG so far is 44.6mpg. However, the figure quoted on the car's computer is 37.7mpg. I was horrified initially as the car was reluctant to claim 30, but it has gradually increased to 37/38 and has been fairly consistent since about 1200 miles.

It does sound as though there is a flaw in the Tuscon's computer calculations, but if other owners rely on it's readouts it might explain why you have had multiple queries. My test drive experience was the same as your other correspondent.
I had hoped to match the WLTP Combined MPG figure of 47.9mpg, but have only achieved that once and am reconciled to 44/45mpg. I intend to raise the issue with the dealer when I get the car serviced in January.