I kinda saw that if you do not assign a plane to a gate then it will give a negative depending on its tier. However if a lot of X/L tier planes arrive to PRG. Then if there are no gates left it will still decrease the level count. I am okay if I don’t assign planes when gates are already clear, but if there are no gates to assign, the reward level will not go down(my opinion to fix this when player planes come.)
I think the issue that you are facing here will be fixed by an idea the devs are considering:
That can also be avoided by not landing every plane immediately and giving planes time to get to the gate, planes are usually generated by gates available before you land a plane
Even if the plane gets into the landing queue after that, if not assigned, it will decrease the level.
There is a 5 mins time and if there are 8 X/L player planes at PRG. Then you have to miss it but if no gate=no decrease.