open the description of some installed flatpak and click "Remove" button in header bar As you can see in screenshot, button label changes to "Downloading" while the package is uninstalled. This behavior does not occur while discover uninstalls a package from Arch Linux repositories.
Created attachment 112441 [details] uninstalling a flatpak
Git commit 0aca22d85f8f8f45957e79a7c6ce4f3787cce724 by Aleix Pol. Committed on 07/05/2018 at 16:24. Pushed by apol into branch 'Plasma/5.12'. Fix status of flatpak transactions M +1 -1 libdiscover/backends/FlatpakBackend/FlatpakTransaction.cpp https://commits.kde.org/discover/0aca22d85f8f8f45957e79a7c6ce4f3787cce724