TelloFpv _never_ initiates an automatic landing (except on RTH land if the option was activated)
Tello's firmware may decide to autoland on different triggers (VPS, WIFI, battery). Some landings can be cancelled (VPS, low battery), and some can't (Wifi obviously, critical battery).
The percentage shown is just an indicator, that
Tello calculates and sends to the app. For various reasons
Tello may miscalculate the initial percentage (based on the voltage at rest) and as soon as you take off and the motors draw some juice the voltage drops much quicker than
Tello expected.
This may trigger an error #13 or an autolanding as early as 30% battery because autolanding on critical battery is initiated not on percentage, but on "flight time left".
The reason for this is obvious:
Tello has to land safely before it runs out of juice.
Now if you had an early
Tello (or like me received an old stock) you'd have an ancient firmware that does not enforce autolanding. I had a
Tello here that could fly until it drops out of the sky. But in the meantime it was updated, due to other firmware problems