This an annoying issue that makes it a real pain when towing and getting 10mpg thinking you have less than 260 miles before refueling. I had asked my Ford dealer why would Ford put a (small) 26gal fuel tank in a gas powered truck designed for towing when my 2011 F150 XLT I traded in had a 32gal tank. Only answer was this 2017 is a cab redesign year maybe they wanted to save weight. Luckily I came across this mentioned on a Ford truck forum and found the TSB 17 -0046 that covered this issue and is covered under emission warranty.

truck showed having 20 miles until empty ( should be about 2 gal)… I top the tank off, should hold 34 Gallons.. I can only fill it 25 Gal.... While I am doing this another 2017 owner pulls up and said he had the same problem. He took it to the dealer and they changed the fuel transmitter free of charge under warranty.

Update from Sep 23, 2019: Dealer told be there is a service buletin on this issue.. a new tank and fuel transmitter has been ordered...