GI_TYPELIB_PATH is not set
GI_TYPELIB_PATH seems to be missing and due to that Builder can't load plugins.
19:52:54.0990 ide-application[ 2]: MESSAGE: Cannot enable Python 3 plugins: Typelib file for namespace 'GtkSource', version '4' not found
Here's a diff with the 3.28 Runtime. Setting it to the same path should be enough fix the issue.
➜ ~ flatpak run org.gnome.Platform//3.30 sh-4.4$ echo $GI_TYPELIB_PATH sh-4.4$ exit exit ➜ ~ flatpak run org.gnome.Platform//3.28 sh-4.3$ echo $GI_TYPELIB_PATH /app/lib/girepository-1.0 sh-4.3$