It is possible that it is as you write (because before, when I did not do this mission, although I do not check it on an ongoing basis during the session, there were trucks on this border), and after doing this mission on the border there are no trucks anymore, in any case, I was up to date with every pumpkin and had already completed 49 before this update 1.0 came out.

When I destroyed the last pumpkin on the border, the achievement was unlocked, but I still only had 49 pumpkins. I couldn't find the last one, so I had to reset the game. When I went through Bratislava according to the instructions, I found only 49 pumpkins, and there is only one on the border, which is on the roof of the border crossing. The pumpkin number 43, which I already found, is not there currently. So I don't know where to find pumpkin number 50, and I have the impression that this is a bug.