I have just discovered that kdenlive when calling kdenlive_renderer it honors system PATH.
It should honor the directory where kdenlive binary is executed at the first place.
I thing is bug or at least is not convenient when you have multiple kdenlive version installed (for tersting reason for example).
You have to struggle with PATH settings etc. this is very error prone.
Some of my rendering may have been wrong due this issue! And probably not only mine!
set PATH in command line session and call kdenlive: