Iwould call a different contractor to service the A/C and inform them of the leak.
After the service tech is finished, ask them where the leak was and how it was repaired.
There are a few bad apples out there that don't take the time to properly recover the refrigerent, pressurize the lines with nitrogen, then inspect all joints with soapy bubbles.
This process takes some time so expect a higher bill then in the past, but you won't be paying for refills every year.