Sorry, I forgot to mention that I am running the game modded, and it may be due to that, I'll try to see if I can replicate the results with the game unmodded.
edit: Managed to replicate the results on an unmodded game, It seems to come into effect when more than 20 planes are placed down, and when you switch between the 3d editing mode and the singleplayer scenario mode a few times.