Discover's CMake doesn't currently warn if the Flatpak dev packages aren't installed (libflatpak-dev on Ubuntu-based distros); instead it silently doesn't build the Flatpak backend libraries.
Git commit 01deaa8cf7b89725c3bc5b59fb8f58069e82774a by Aleix Pol. Committed on 28/12/2017 at 16:20. Pushed by apol into branch 'master'. Warn if building flatpak is enabled but some dependencies aren't found M +5 -0 CMakeLists.txt M +2 -0 libdiscover/backends/CMakeLists.txt https://commits.kde.org/discover/01deaa8cf7b89725c3bc5b59fb8f58069e82774a
Confirmed, thanks for the super quick fix!