Gnome Software and Discover not installing third party RPMs


I am currently trying out openSUSE tumbleweed in two virtual machines: two fresh installs, one with a Gnome desktop and one with Plasma desktop. I have tried using gnome-software and discover on their respective desktops to install third party RPMs that I had manually downloaded. However, it seems both are unable to do so (which make it look like it may be related to packagekit itself). From nautilus/dolphin, opening the RPM in with gnome-software/discover does nothing. From the command line, gnome-software has a --local-filename option to open a local package file, which opens up gnome-software saying “The file is not supported”.
Installing packages from the official repositories that are listed by gnome-software/discover, on the other hand, works perfectly.
The problem does not come from the RPMs themselves I believe, as YaST can install them just fine and they seem to work properly when installed.
The RPMs I tried to install are for Visual Studio Code and Ghetto-Skype.

I know openSUSE has YaST for package installation, but it seems weird that those two packagekit applications seem to not be able to install third party RPMs, while I was able to do that in Fedora for example.
I have no idea if this could be related, but something similar happened in Ubuntu a few months ago:

And I forgot about package versions:
gnome-software 3.22.1-1.1
discover 5.8.3-1.1
PackageKit 1.1.4-1.2