I this GNOME gitlab issue a GNOME dev states that he doesn't expect the behaviour of the background applet to change soon.
Also keep in mind that GNOME unfortunately tends to default to Flatpaks. Some core apps are shipped as flatpak as default, and also GNOME software center defaults to prefer flatpaks over native distributions packages!!! (Which can be changed via dconf). And which Solus devs have changed by the way in the GNOME Software Center they ship with Solus, just to be clear on that. Though to prefer flatpaks is kind of hardcoded in the GNOME software center. Even if you change settings to prefer distribution packages, for a fraction of a second you see that GNOME software center shows the flatpak package first, before it switches to eopkg packages. Anyway, I think even the idea sucks to prefer flatpaks over distribution shipped packages, and I also whish they would make background applet to also show non-flatpak apps, because otherwise it's quite useless, frankly.
But you know, GNOME being GNOME, I don't expect much in that regard.