Actually by using such a high gear, it should be the opposite.
Hit a high wattage floor, which means that you have to lower the cadence in order to hit lower watts.
Besides the above, using high gears, un-smooths the power that the trainer can hold and the output should look very jagged.
This happens because it cannot calculate power correctly with very high flywheel speeds.
This makes me very skeptical and in this case i would suggest, as mentioned, a calibration and afterwards check the nearby interference from other devices.
Disable bluetooth on all other devices.
Make sure that your trainer is not connected to your head unit as controllable. This will make it uncontrollable for the suf app.
As @JamesT mentioned, use the small ring and a mid cassette cog and only increase gears if you cannot hit the specified watts, as i mentioned before.