This is because su - does not change the XDG_RUNTIME_DIR environment variable.
Setting XDG_RUNTIME_DIR=/run/user/ja is not going to solve the problem, since dconf relies on the system to create (and remove) the runtime dir.
Unsetting XDG_RUNTIME_DIR will make dconf use ~/.cache, and thus avoid the problem.
This is because su - does not change the XDG_RUNTIME_DIR environment variable.
Setting XDG_RUNTIME_ DIR=/run/ user/ja is not going to solve the problem, since dconf relies on the system to create (and remove) the runtime dir.
Unsetting XDG_RUNTIME_DIR will make dconf use ~/.cache, and thus avoid the problem.