Unable to install flatpak from flathub.org via the "Install" button (flatpakref not working)

Steps to reproduce:

- visit https://flathub.org
- find a package, for example: https://flathub.org/apps/com.mastermindzh.tidal-hifi
- Click the `Install` button.
- This downloads a `.flatpakref` file.
- Open the file (some browsers have a direct "open" option that opens the file after temporarily downloading it, but you can also double click it in Gnome Files or similar).
- This causes the `Add/Remove Software` app to open, but nothing happens (or there's no indication that anything is happening).

I expected that opening the `Install`ed (downloaded) file would cause the flatpak to be installed by `Add/Remove Software` (Pamac).

When `Add/Remove Software` opens, we end up on an empty view like the following, with no output, no loading animation, nothing happening:


Reproduce the issue on Manjaro by opening a Flathub .flatpakref through Pamac's Add/Remove Software app. Start by tracing how Pamac handles the downloaded file and determine why the view remains empty; done when the package installation starts and provides visible feedback.

Written by the indexing model from the issue text.