Oh okay ! Would be nice if someone working on the raspberry pi's firmware could investigate this. Maybe it's due to the screen controller, in that case i'm out of luck. If it's on the GPU side maybe there is something they can do, as this white screen (seems to) serve no purpose.
The display is TN technology --> it's normally white.
If there is no pixel data when the backlight is switched on the display is white. That's why many displays have Backlight Enable signal to prevent irregular display. Backlight Enable will be set after valid pixel data is transferred.
So maybe somebody at RPi Foundation needs to check the DSI power on sequence.