When serialising JSON with float/double values, there is control on how many digits to write after the decimal point, with default of 2.
However, if the number is bigger than 1000 or smaller than 0.001 it will always be serialized with exponent: 1234.5 to 1.23e7

I can track this hard coded behaviour to: https://github.com/bblanchon/ArduinoJson/blob/master/include/ArduinoJson/Serialization/JsonWriter.hpp#L95

This is not in line with Arduino Serial.print that does not add exponent.