If the torque converter drain plug is tight and not leaking the only other places it could leak are the front seal and the front pump seal. To fix either of the seals requires transmission removal. The front seal is an easy job, the front pump seal requires disassembly of the front part of the transmission (pump, stator, etc). The torque converter is very sturdy, I would not think it'd be punctured. I'd take pictures if you can and call the service manager. Do NOT accept this leak as "it might happen with fluid changes" or age related. If one or both of those seals are leaking they were disturbed outside the scope of a fluid change!