This is something I noticed also, along with MenuItem Icons showing up; when I was testing on linux.
As far as the menuIcons, I recall there was a OS setting somewhere that regulated this stuff.
So maybe the showing of MenuItem background colors might be similarly related.
For example, maybe your Linux distro is overriding them…
Can someone/others on/with Linux test poster’s OP attachment and report back if this works for them or not…?
I can’t seem to on kubuntu/mageia (or at least can’t find the settings to change menu behavior).
Other distros/settings…?