starting Gittyup from the icon ignores the system proxy settings

I've Gittyup 1.2.2 installed via Flatpak on Deepin 20.8 (Debian) behind a corporate http proxy trying to access some remote repositories (gitlab and github).
Starting Gittyup from the icon ignores the system proxy settings and I can't reach the repositories.
However, running Gittyup from the command line with flatpak run com.github.Murmele.Gittyup works.

I've installed flatpak with

sudo apt install flatpak
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
sudo apt install gnome-software-plugin-flatpak

and then gittyup with

flatpak install flathub com.github.Murmele.Gittyup

output of flatpak list:

Name                               Application ID                                 Version          Branch              Installation
Gittyup                            com.github.Murmele.Gittyup                     v1.2.2           stable              system
Mesa                               org.freedesktop.Platform.GL.default            21.3.9           21.08               system
Intel                              org.freedesktop.Platform.VAAPI.Intel                            21.08               system
openh264                           org.freedesktop.Platform.openh264              2.1.0            2.0                 system
Adwaita theme                      org.kde.KStyle.Adwaita                                          5.15-21.08          system
KDE Application Platform           org.kde.Platform                                                5.15-21.08          system

The proxy is enabled system wide in control center via auto configuration url, and HTTP_PROXY HTTPS_PROXY variables are set in user's .bash_aliases.