This should be resolved now.
Yes, you rock !
it works with this change
indeed in the file /opt/retropie/supplementary/runcommand/runcommand.sh
Now the video mode selection is working great
The mesa-drm package should not be needed for a RPI3b+ installation.
Please add more info about your setup - as detailed in https://retropie.org.uk/forum/topic/3/read-this-first.