Answered by email.
Unfortunately, I can do nothing about it because it’s an OpenWeatherMap limitation.
More info here: https://openweathermap.org/forecast16
“Please, do not confuse min/max parameters in current weather API and forecast API. In current weather API temp_min and temp_max are optional parameters mean min / max temperature in the city at the current moment to see deviation from current temp just for your reference. For large cities and megalopolises geographically expanded it might be applicable. In most cases both temp_min and temp_max parameters have the same volume as ‘temp’. Please, use temp_min and temp_max parameters in current weather API optionally.”
To summarise, only 16 days forecast API has min-max temperatures. And it’s not include with the FREE key.