What you say makes sense and bullet creep is definitely an issue with light revolvers. I think we had three issues all mixed up.
@Charlie_K was on the right track.
This is a direct quote from S&W.
“Hi, the reason why we recommend you don't shoot .357 magnums under 120 grain in that gun is because the quick burning powder that is associated with the lighter grain bullets presents the possibility of eroding the face of the titanium cylinder. This only applies to the .357 magnum round, below 120 grain is fine for the .38 special”
So it wasn’t the top strap like I thought but the cylinder face. And apparently it happens in only a few shots.
My 340pd has a stainless shield built into the top strap to prevent flame cutting.
Certainly not trying to beat a dead horse, just setting the record straight sorry for the mis information.