If you start the krita flatpak (org.kde.krita) from flathub and try to open the file open dialog while running KDE, nothing happens. If you do same under GNOME, the dialog pops up. If you delete /run/user/1000/flatpak-info in the sandbox, the dialog also shows up when running under KDE. Maybe a bug in xdg-desktop-portal-kde ?
By the way: krita is the only Qt app I encountered, that shows this behavior.
I think I found the problem, I'll submit the fix to Qt and backport to KDE runtime.
See https://codereview.qt-project.org/#/c/229189/.
Fixed in Qt and backported to KDE runtime. Once the runtime is build in Flathub, you will receive it as update.
I can confirm that this fixes it for krita 3.3 (the version on flathub at the moment). But I'm in the process of updating it to 4.0 and there it's broken again.
Is it possible to try version 4.0 somewhere?
Krita 4.0 is now on Flathub.
This works in Krita 4.1.0.