Force 4" MPI4008 to 720x1280 definition

[watchever]

I have installed the MPI4008 4" screen on Rpi5 Bookworm and want to force the resolution to 720x1280.
It works on Rpi4 Bullseyes by updating the config.txt file with following :
hdmi_cvt 720 1280 60 6 0 0 0

I know it is advertised as 480x800 screen but it is a very good point to be able to force to higher definition.

Any idea why it is not working on Rpi5 Bookworm and how I could solve it ?

Thanks for your help