This happen on all airports doesn't matter I had changed the positions of the loader or not. This also just happend with deboarding.
So it's a problem of the airplane configuration you are using.
If you use only the right cargo doors, they must be cargo 1 and cargo 2, and will be used only by loaders that (by default), starts on the right side of the parking, which can of course customize (but not place them on the left side of the parking...)
If you use a main cargo door on the left as well, it must be the "cargo main" on the airplane editor, and it will used by a "cargo main" loader that starts on the left side of the parking, which you can customize as well.
If you use a plane with a cargo door on the right another on the left (but not a main door), like a CRJ, the one of the left should be cargo 2, and it will be used by a loader that starts from the right, will turn around the tail and will reach the door on the left.
In all cases, be sure the door are configured properly, including their orientation, which is the green arrow on the airplane editor, that MUST point in the OUTSIDE direction.