Swapping is currently not allowed on a trip once a person has been denied a position on the trip due to the green-on-green restriction.
When a person is denied a position on the trip due to the green-on-green restriction, the system freezes all existing awards of other positions on that trip (on senior lines). This is done to avoid seniority violations. For example, if Person A (green) is denied a position on the trip due to green-on-green (with person B (green) holding another position on the trip), the system freezes all senior-held positions on the trip. This avoids the scenario where the position held by person B later gets moved to a different (non-green) person, and then a more junior person (e.g. person C (green)) is able to take the trip, causing a seniority violation (because person A was previously denied the trip).
In this case (and in most cases), probably the system would have considered the free position first (because the system generally favors considering free positions first). Because that free position would have been denied to the junior due to the green-on-green restriction, that would have triggered the freezing of existing awards on senior lines, which would then have prevented the same junior from taking the other position through a subsequent swap.