I am sure you read in the manual "Maximum time in milliseconds that the jump strength is sustained at while the jump control is being held before the effect of gravity takes over." I read that one once myself too !.
The thing is that "jump control is being held". It only knows wich one the jump control is when you use default controls.
I suppose you dont ?
If using default controlls and setting the time to like 300 miliseconds, the effect cant be overseen.
If you dont use default things, you gots to code it all yourself.
One easy way is this:
https://www.scirra.com/tutorials/1048/p ... k-duration