RC TRIM not working with V tail mode

Hello All

I use V tail mode on my drone.
The problem is I am not able to set neutral position for each control surface (I use a custom RCMAP with elevator on CH3 and rudder on CH4)
When I switch from manual (where neutral is set by my tx) to FBW or RTL, the control surface neutral position not respond when I change RC TRIM…

Actually to correct this problem I use the TRIM_AUTO function, but I would like to understand why control surface not move with RC TRIM.