Exporting .apkg and .colpkg in Linux Mint doesn't work

Hi @ all.
I’m new to Anki and noticed some strange behaviour.
I’m using a clean install of Linux Mint 20.2 and downloaded “Anki (Flathub)” as a flatpak from the build-in software center. In general Anki seems to work as expected as far as I can tell but I have limited practice with Anki.

The software itself says I am currently running Version ⁨2.1.44 (b2b3275f)⁩.
I will provide detailed debug information at the end of this post.

When trying to export files as a local backup the default directory is /home/username/.var/app/net.ankiweb.Anki/data/Anki2/User 1/collection.media/
The directory is empty and if I click on ‘Save’ the Nemo window (default file browser on Linux Mint) closes and Anki gives me an error message saying “Please choose a differen export location”.
Next I try to save the backup data on the desktop but it seems like I cannot change the path to /home/username/Desktop. Instead it’s only possible to navigate through the directories of the first path (/home/username/.var/app/net.ankiweb.Anki/data/Anki2/User 1/collection.media/).
Even weirder when I choose /home/username/ (also known as /~ in Linux) it seems to save the file as intended but when looking for that .apkg or .colpkg file I cannot find it using the default file browser nor can I see it in Terminal when running ls -a (lists all files / directories in the current path including hidden files and directories). The funny thing is that when I try to export the file again Anki shows that the data is already in the location. It even warns me to overwrite the initial export. After restarting the machine and trying to import the backup data Anki also cannot find any data.

So for the moment I will use the online synchronisation but overall I would like to use local backups and at the moment I cannot see how.

I even created a second install of Linux Mint and Anki on a second drive and I could reproduce the same behaviour as described above.

Here is the detailed debug information:

    Anki 2.1.44 (b2b3275f) Python 3.8.1 Qt 5.15.1 PyQt 5.15.1
    Platform: Linux
    Flags: frz=True ao=True sv=2
    Add-ons, last update check: 2021-07-26 14:05:12
    ===Add-ons (active)===
    (add-on provided name [Add-on folder, installed at, version, is config changed])
    Advanced Browser ['874215009', 2021-07-22T23:14, 'None', '']
    Hierarchical Tags 2 ['594329229', 2021-03-07T03:25, 'None', '']
    ReviewHeatmap ['723520343', 2019-08-03T17:14, 'None', '']
    True Retention ['613684242', 2017-11-19T21:43, 'None', '']
    ===IDs of active AnkiWeb add-ons===
    594329229 613684242 723520343 874215009
    ===Add-ons (inactive)===
    (add-on provided name [Add-on folder, installed at, version, is config changed])

Please see Third party Linux packages and source builds are not supported - Frequently Asked Questions