The only code I can think of off the top of my head that might indicate a leak would be a lean condition code. Since their repair didn't resolve it, I suspect the issue to be a faulty mass air flow sensor. It's very common to get a lean mixture code when this fails.
Sometimes you can unplug it and then test drive the car to see if the misfire is still there. It basically puts the computer into a default mode. If it's running better, then I would replace the maf sensor.
This is the sensor bolted to the air filter box on the car. Unplugging it for testing will store a code, but since the light is on already it won't matter.
If you want to give it a try and see if it's any better, it's not really a bad test. Otherwise, I really should have the code to give you more detailed info on it.
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Thank you so much!