I think I have found an issue. Can you reproduce it?
Open the Plasma 5 System Settings. Go to Workspace Theme / Desktop Theme. Click on Get New Theme, select one (I tried this with the most recent ones) and click Install.
The intended behaviour at this point is that the new theme should be in the list of available desktop themes. But, on my machine, it isn't. Restarting the System settings makes no difference. The theme now has an "Uninstall" button next to it under Get New Theme, but that's all.
The workaround of downloading and extracting the theme manually (into ~/.local/share/plasma/desktoptheme) works. But the System Settings don't function as intended.
I am using Mageia 6 on a Lenovo ideapad 310.