If I remember correctly (as its been a couple years since I last played), all land units except PARA remain stationary for 24 hours after completing mission. The fact that a PARA just landed (and is locked from being able to receive a move mission for 24 hours) but can literally be "sucked up" by a fresh TRA loading him is the problem. Note PARA does not move to TRA; rather TRA loads him. And that is the error making possible the incorrect very frequent re-use of PARA being discussed in last several posts.
But as regards the opening of the thread, I would say that any PARA that can manage to take the province deserves it. It is extremely cunning I think to commit a para to an attack, then halt that attack by retreating everybody... and then with incredible timing land a fresh PARA. Sounds like the enemy got fooled.
Merry Christmas 2020 everybody!