Both Appimage as well as Snap packages seem to lose the destination file path somewhere along the way, meaning that ffmpeg errors out. Snap packages on top of that have the problem that they cannot get to usr/bin, so they can potentially not select the default installed ffmpeg. Lanchpad and self-compiled Kritas don't seem to have this issue.
This patch fixes the problem for AppImage and the first part of Snap's problem: https://phabricator.kde.org/P61