Wrong framebuffer dimensions when downscaling monitor resolution

[Crystal4276]

 * mutter 5.6.1
 * Distribution - Arch
 * RTX 2080, 525.60.11 (proprietary)
 * 64 bit

Issue
Triple monitor setup:

• Two identical screens with native resolution 3840x2160
• One wide screen with native resolution 3440x1440

I want to scale the two 4K monitors down to 2560x1440.
No option for this in the Display GUI app (the resolution is not provided by EDID), so i'm using xrandr instead :

xrandr --fb 8560x1440 --dpi 110 \
--output DP-0 --mode 3840x2160 --scale 0.666666667x0.666666667 --pos 0x0 --panning 2560x1440+0+0 --rotate normal \
--output DP-2 --primary --mode 3440x1440 --rate 100 --pos 2560x0 --panning 3440x1440+2560+0 --rotate normal \
--output DP-4 --mode 3840x2160 --scale 0.666666667x0.666666667 --pos 6000x0  --panning 2560x1440+6000+0 --rotate normal \

This command is working perfectly in other windows manager like qtile.
In cinnamon/mutter, the framebuffer is not adjusted to --fb 8560x1440 as requested.
Instead we get :

 Screen 0: minimum 8 x 8, current 9840 x 2160, maximum 32767 x 32767

This cause problem since the cursor reaching the bottom or right of the Xscreen will cause a desktop scrolling.
Note also that configuring the monitor downscale with nvidia-settings/xorg.conf lead to the same issue (only in cinnamon again).

Steps to reproduce
Cf. above but in essence, use an xrandr command to downscale a monitor resolution while adjust panning/framebuffer dimensions accordingly.

Expected behaviour

The framebufffer dimension / panning should be correctly picked up by the cinnamon.
Eventually the display GUI app could also propose an option to downscale monitor resolution (when not listed by EDID).

Other information
Illustration of the problem