That appears to be the difference. In the logs below you can see I'm getting 0.0 instead of null. I didn't realize you were actually getting null back. If I hit the API endpoint outside of Hubitat here are my results:
"tpms_pressure_fl": 0,
"tpms_pressure_fr": 0,
"tpms_pressure_rl": 0,
"tpms_pressure_rr": 0,
I'm in a Model Y on 2022.4.5.21 a912b0d64483
which is the most recent FSD beta, so if you're on another version, maybe that's the difference.
Not Driving:
[dev:676] 2022-04-23 08:40:17.890 processData: [state:online, motion:inactive, speed:0, vin:5YJYGDEE1MF123456, thermostatMode:off, chargeState:[battery:80, batteryRange:248.14, chargingState:Complete, chargeLimit:80, minutes_to_full_charge:0], driveState:[latitude:40.766683, longitude:-73.980743, method:wgs, heading:113, lastUpdateTime:Sat Apr 23 02:00:07 EDT 2022], vehicleState:[presence:not present, lock:unlocked, odometer:13082.020376, sentry_mode:Off, front_drivers_window:Closed, front_pass_window:Closed, rear_drivers_window:Closed, rear_pass_window:Closed, valet_mode:Off, tire_pressure_front_left:0.0, tire_pressure_front_right:0.0, tire_pressure_rear_left:0.0, tire_pressure_rear_right:0.0], climateState:[temperature:50.9, thermostatSetpoint:69.8, seat_heater_left:0, seat_heater_right:0, seat_heater_rear_left:0, seat_heater_rear_right:0, seat_heater_rear_center:0]]
Driving:
tire_pressure_front_left:3.2, tire_pressure_front_right:3.2, tire_pressure_rear_left:3.15, tire_pressure_rear_right:3.175