SUMMARY
Aurora is an immutable distro based on Fedora Kinoite. In both distros, /home is a symlink to /var/home, because of this the flatpak-integrator fails to install the plasma-browser-integration-host into all flatpaks.

STEPS TO REPRODUCE
1. Make a symlink to your home directory.
2. Run kded6 --replace with $HOME set that it points to the symlink.

OBSERVED RESULT
flatpak-integrator doesn't install the integration.

EXPECTED RESULT
flatpak-integrator should install the integration.

SOFTWARE/OS VERSIONS
(available in the Info Center app, or by running `kinfo` in a terminal window)
Linux/KDE Plasma: Aurora 42
KDE Plasma Version: 6.4.0
KDE Frameworks Version: 6.15.0
Qt Version: 6.9.1

ADDITIONAL INFORMATION
This can be work around by setting HOME to /var/home/user, but it's not the default in these distros.

Looking at the code, this seems to be expected behavior? Couldn't find what's the reasoning behind it though
https://invent.kde.org/plasma/plasma-browser-integration/-/blob/master/flatpak-integrator/plugin.cpp?ref_type=heads#L199

Also, here's the logs shown in the console

org.kde.plasma.browser-integration.flatpak.integrator: Failed to open hostWrapper directory. "/home/santi/.var/app/org.mozilla.firefox" : unknown error
org.kde.plasma.browser-integration.flatpak.integrator: Failed to open hostWrapper directory. "/home/santi/.var/app/io.gitlab.librewolf-community" : unknown error
org.kde.plasma.browser-integration.flatpak.integrator: Failed to open hostWrapper directory. "/home/santi/.var/app/com.google.Chrome" : unknown error
org.kde.plasma.browser-integration.flatpak.integrator: Failed to open hostWrapper directory. "/home/santi/.var/app/com.google.ChromeDev" : unknown error
org.kde.plasma.browser-integration.flatpak.integrator: Failed to open hostWrapper directory. "/home/santi/.var/app/org.chromium.Chromium" : unknown error
org.kde.plasma.browser-integration.flatpak.integrator: Failed to open hostWrapper directory. "/home/santi/.var/app/io.github.ungoogled_software.ungoogled_chromium" : unknown error