I don't know if it can help because you the path should be known by Lpub but, Lpub created a "model/LPub/parts" directory where there are only the Ldraw parts and not the Lsynth parts of the model.
As you asked in your previous message, I tried to move the Lsynth part files to this directory, but doesn't work.
As you asked in your previous message, I tried to move the Lsynth part files to this directory, but doesn't work.