Trying to get aquainted with DT on Linux, I found that my own lua scripts only get loaded, when I start DT from the config directory, where my luarc resides.
If I start it the from the gnome programme starter, my scripts don’t show up.
Try starting darktable from the command line with darktable -d lua and see if that gives us a clue.
Also, try whereis darktable and see if the executable is installed in two places. I’m not sure why starting it from the command line starts in a different directory than from the launcher.
Ubuntu is ‘Bionic Beaver’ and ‘Focal Fossa’ - tested on both with identical results.
The output of ‘whereis darktable’ on both machines is darktable: /usr/bin/darktable /usr/lib/x86_64-linux-gnu/darktable /usr/share/darktable /usr/share/man/man1/darktable.1.gz
My luarc-file is in $HOME/.config/darktable
I have a print-statement in luarc, which I see only when starting darktable in the terminal in the directory $HOME/.config/darktable
If I start darkable from a terminal in my homedirectory, the output of the print statement never appears.
As it is to be expected, my lua-extensions only show up when the application is startet from $HOME/.config/darktable.