Screen customization : default planning mode not set

Hello,

i use screen customization to set default planning mode for activities.
I set value to "8" according to planning modes to get "fixed duration".
 

I close all planning modes for activity other than "Fixed duration" :
 

But, when creating new activity, i get "As soon as possible" as default value and two possible values :


My purpose is to mask planning mode for creating new activity to force using "fixed duration" and simplifying to user's screen but it is not possible while default value set in screen customization is not used...

Thanks for your work !