The Magic Portals work with a collision prim that detects when the avatar walks in to the portal. For realism this collision prim is located inside the portal so that when people walk against the sides or other parts of the portal they don’t get teleporter.
With some portals such as the 4D Portal Door the collision prim is located behind the door, so that you see your avatar walking through the door before they get teleported. This may be a problem when you put the door on a wall as the collision prim might result in the wall.
As all the Magic Portals are mod you can easily fix this if required. Just edit the portal, select “Edit linked” and find the prim with the “FW COL” script in it. Reposition this prim to where you want and done.