Try shooting it with 124 or higher first to see if the guy at Glock was right about YOUR specific gun. Many can shoot 115 with no issue I know it would suck if your Glock was one that could only shot 124 or higher without BTF issues.
I would do some checking on a gunsmith before giving him my gun. How reputable is he and does he have a money back or warranty or insurance on his work, many places that do work is bring it to us at your own risk. Lets say as an example only
he does something like cutting into the frame (which voids factory warranty) to lower where the ejector sits and then you still have the same problem or worse yet. Is he an accredited gunsmith or at least a Glock Armorer. In every occupation there are good and bad workers.
If he claims he's only messing with the ejector make sure he is only messing with the ejector or find out exactly what else he touched in case something else goes wrong. Be careful with magical fix all claims. Someone saying they get new Glocks in all the time and fixes them 100%, is he the only guy in the states that can fix this issue. Either way, I wish you the best and hope your problem gets fixed.
It would suck if you had a lemon/rare unfix able or maybe you offended some person on your block that studies witchcraft cursing your Glock forever.