Journey Planner - Param walkingSpeed Doesn't work


I am testing the Journey Planner API for my project and it looks to me that walkingSpeed parameter only works for the walking mode.

For example, for N7 0NL to EC1V 9QP, walking mode, under different walking speed, I got different duration 55min, 72min, 103 min, which is OK.
But under public transit mode, I always get the same duration 42min. I checked the data against Journey Planner website, and can see 49 min for slow walking speed, 42 min for average walking speed, and 34 min for fast walking speed.

Any idea how to fix it?