Something weird happened: One of my flights assigned to medium stand changed from medium aircraft to small aircraft, as you can see in the picture below.
Wait until you see it the other way around, on a stand, in stead of in the planner
For your issue, make a bug report to have @Olof take a look.
this bug has been in the game a while, the auto scheduler can schedule medium flights on small stands
You can also “accidentally” drop a medium flight into a small stand in the flight planner if you click reschedule then move the mouse up and down quickly between small and medium stands you can drop it onto a small and it will land there ETC as normal.