I weld in a mixed environment with aluminum, steel, stainless, and grinding. The welding is mostly flanges, bosses, or other tubes to tubes, using turntables. I would say your leaking issue sounds like dirt, dust, technique, or filler metal.
I recommend the parts be clean. Don't tack a skid of parts and let them set for a week, next to a grinding station, until their welded. I sometimes use acetone to clean the part just prior welding.
The start stop point is also an issue. When I GMAW stainless tubes to flanges or tubes to bosses using my robot, I overlap the start stop points by at least .75. I also turn up the heat at those points. Torch angle could also be an issue. 45 degrees straight on, or pushing up to 15 degrees works well.
Another point. I had a stainless flange to tube and boss to tube job that leaked all the time using 308 rod with my robot. I switched to 316 rod to stop leaking.
Good Luck!