I had also this issue in my Jolla1 with some already previously installed plugins showing not installed / available for download, even after restarting the application (both UI and server processes).

I took a backup of .local/share/harbour-situations2application//harbour-situations2application/situations2/ folder contents and then removed featureindex.json, plugins.json and features/* from the folder, and restarted the app. Now those previously badly behaving plugins could be installed just fine.

I even went as far as taking another backup with all installed plugins showing correct status in settings and testing if reverting back to the original files from the backup brought back the original status (it did), and if restoring from the later "good" backup fixed the issue (it did).

So, if you Heikki want to have a good-bad pair of that folder for comparison, I can provide that by email.