File chooser dialog cause flatpak applications to hang in KDE

Flatpak seems to have serious issues on kubuntu 20.04, at least with some applications that immediately hang when a file dialog is shown.

One of them that makes the issue well reproducible for me on kubuntu 20.04 is shotcut (https://www.flathub.org/apps/details/org.shotcut.Shotcut). Just install it and then press the "Projects Folder" button in the "New Project" to make the app hang.

Trying the more recent flatpak on the ppa:alexlarsson/flatpak ppa, which is at 1.10.2 provides a slightly less catastrophic outcome, since the app doesn't hang. However the "open" button of the file dialog does not work and the dialog cannot be closed.

The issue may have to do with the xdg-desktop-portal-kde rather than flatpak itself.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: flatpak (not installed)
ProcVersionSignature: Ubuntu 5.8.0-45.51~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-45-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: KDE
Date: Sun Mar 21 22:13:27 2021
EcryptfsInUse: Yes
InstallationDate: Installed on 2020-02-16 (399 days ago)
InstallationMedia: Kubuntu 19.10 "Eoan Ermine" - Release amd64 (20191017)
SourcePackage: flatpak
UpgradeStatus: Upgraded to focal on 2020-05-23 (302 days ago)