Sightseeing seems to have an effect created for it. This effect is not applied to duplicants who were in a sightseeing module in a rocket that just came back.
The stress-reducing effect of sightseeing can be replicated by having a duplicant enter the module and then immediately come out. This will reset stress to 5% and remove existing morale effects.
Not sure which one is a bug here since the description is ambiguous.
Launch a rocket with a duplicant in a sightseeing module. Wait for it to return. Have another duplicant come into the sightseeing module and then immediately de-assign them. Compare effects on two dupes (one who saw the cosmos, other who didn't).