and indeed, the problem see to be the DBusActivatable=true in the desktop file
a workaround would be to copy /var/lib/flatpak/app/org.telegram.desktop/current/active/export/share/applications/org.telegram.desktop.desktop to ~/.local/share/applications/. and removing the line DBusActivatable=true from it
Sorry for making a fuss… (should have started searching from this discourse, not from google…)
at least maybe this post help someone else googling specifically for telegram
hope nixos’s issue will be fixed soon so there won’t be a need for the workaround