The spring bolts allow some movement between the pieces, if they are hard locked together, it puts a lot of strain on the parts.
My son had a shop replace the power steering pump in his Matrix, and they left a motor mount disconnected. Put the car under a load, then engine would shift enough where one of those spring loaded joints would open up. Made a hell of a racket. found it by having one person stand hard on the brake pedal so the car wouldn't move and hit the gas in gear, another standing to the side (out of the way
) watching the engine. If really flopped about.
Also, if the mechanic replaced the one gasket, its possible that a different joint got flexed and weakened.
a really sketchy way to find out if its that joint between the cat and the muffler would be to tightly wrap something like an old towel around the joint, tie it on tight. Something to act as an external gasket, muffle the leak. If that's where its coming from, it will sound different. Obviously, you would want to be sure to do it only when the engine and exhaust both cold, and only run that way for a really short period of time. Also Obviously, if you do something like that and set your car on fire you can't blame me if you take insane advice from some random nutjob wacko on the internet