The eighteenth arcanum is about deception as a way of living. Not about a single lie told to spare someone, but about a constructed picture in which the people nearby went years without knowing what was actually happening.
In the lineage this comes back as the descendant not being believed and being deceived themselves. The truth said plainly arouses suspicion; second meanings and hidden circumstances keep turning up all around; a habit appears of leaving things unsaid "so as not to complicate matters" — and that same habit reproduces the law.
The working-off is to tell the truth where a lie would be safer and more convenient. Especially the inconvenient truth, and especially about oneself. This also covers not supporting someone else's deception by silence: keeping quiet counts here as taking part.