What is happening for modify_duration?

[thisiscam]

I was trying to modify duration for a sub-duration of a signal, but realized that the synthesizer doesn't generate good quality output. I then noticed this line in the example: https://github.com/tuanad121/Python-WORLD/blob/master/example/prosody.py#L44 and the modify_duration function, which seems to be exactly what I wanted: https://github.com/tuanad121/Python-WORLD/blob/master/world/main.py#L177

Seems like the current code fails on this operation? Is there any understand why this is happening?