Astronaut after arriving from a mission will look un-suited, even when actually wearing a jet suit. The dupe can still fly and will dock the suit later on, it's just invisible until the dupe removes it.
EDIT: Earlier bug report was incorrect, the dupe REALLY doesn't lose the suit, but the bug triggers even with gantry
Send jet-suited astronaut into space. Save+load (may not be necessary?). Wait for the rocket. Deconstruct the capsule or build a gantry. Look at the dupe.