Frost is forming on one of the condenser tubes near the bottom of the condenser.
This problem is caused by a restricted refrigerant passage in the condenser. When frost is forming on one of the condenser tubes near the bottom of the condenser, the refrigerant passage is restricted at that location in the condenser.
Frosting is a clear indication that there is a restriction in the condenser.
Airflow restriction. he condenser.
Restricted airflow passages in the condenser would not cause frosting; it would limit the efficiency of the system because the refrigerant would not be cooled by the air flowing through the condenser.
A leak would not cause frosting either; oil may be seen at the point of leakage.