What I understand is that any convoy in Poland hit would be an act of war. Once over the border , its potentially fair game.That Putin has directly threatened deliveries of munitions to Ukraine as a legitimate target could be a turning point.
I can imagine the intent being to attack a delivery, but the aircraft go over the line and fly over a NATO country. NATO could then attack the aircraft and, if they flee over Ukraine, use it as an invitation chase in hot pursuit.
I'd not like to predict the outcome.