About what date was that drop? Firmware? I saw something similar all of a sudden in mid-April nearly in sync with getting 2019.8.5.
Went from 2018.50.6 to 2019.5.15 on 3/13/2019.
The last charge on 2018.50.6 calculated 308.8 miles at 100%.
The first charge in 2019.5.15 calculated 314.3 miles at 100%.
TeslaFi range calculations are all extrapolated from what the car is reporting at a given SoC.