Filled up my 2015 for the first time and when I got inside was surprised to see the gauge read just above 3/4's full, is this normal?
I did fill up on a slight incline, but that should not make much difference or is the gauge not very accurate?

 

Mine reads full correctly.
One time I did make the mistake of thinking it was under filled because I had the gauge set up to read differently.
You might check to make sure that you have the full width fuel gauge setting.

 

Known issue with the filler neck. There is a TSB to address it. At certain gas stations, it will click it is done way to early. At those gas stations, pulling the filler in upside down makes it work better.

 

AHAH! So that's it! And I had thought that it was because the car was objecting to that cheaper-than-other-stations-around gas station.

Anyway, to reply to the OP's question.... It happens.

 

I found the TSB:

JTB00443NAS2

SECTION: 310-01
Fuel Tank Difficult to Fill
AFFECTED VEHICLE RANGE: F-TYPE (X152)
Model Year: 2014-2016
VIN: K00001-K32543
Manufacturing Plant: Castle Bromwich
MARKETS: NAS
CONDITION SUMMARY:
Situation: The fuel tank may be difficult to fill, with the fuel delivery 'cutting out' several times. Cause: This may be caused by an insufficient flow rate of the fuel tank filler pipe.
Action: Should a customer express this concern, follow the Service Instruction below.
PARTS:
T2R21271 Fuel tank filler pipe - Convertible Quantity: 1
T2R21272 Fuel tank filler pipe - Coupe Quantity: 1
C2D38206 Fuel tank filler pipe cap Quantity: 1
T2R21273 Protection shield Quantity: 1

 

I had this occur a few times with my 2018 model but did not find it annoying to the point of having it fixed. All too often my dealership does not apply the "First, do no harm" principle of remediation.
I can't recall that it's happened with my 2022 model.

 

Note: The filler neck TSB will help if you're having trouble completely filling the tank because the pump cuts off prematurely. It doesn't affect the gas reading/accuracy. It was a common topic on the forum in the early years, but hasn't come up much lately. For whatever reason, not every car had the problem.