Stainless is not inherently weaker metal, but as some have said, it's a lot easier to harden than to heat treat. It requires a more complicated heat treat than plain old tool steel and WAY more precise temperature control and control over rates of cooling. Cool it too fast, and it's hard--like glass or slate.
Now, all my heat-treat experience is with carbon steel for blades and tools, nothing to do with guns and I don't use stainless, but I just don't think the grain should be anywhere near that large. That just can't be a proper heat treat. For all I know, though, that might be the result of a casting mistake. I know absolutely nothing about casting steel.
This is how 440C knives got a bad name. People were using 440C, which is a stainless high in Chromium, without learning to heat treat it correctly. Predictably, that meant brittle knives that didn't hold up well, and a lot of serious users dismissed "that stainless stuff" and went back to carbon steel. With the right heat treat, 440C can be very tough and still hard enough to hold a pretty good edge.
Maybe if this is a 2001 problem they can dip into the spare stainless slides from older production. Surely the parts department and custom shop have some set aside.
It's always possible, now that I think of it, that a bad casting left a cold shut or some other kind of inclusion at that point in the slide. If it were a casting problem, that would explain why it happens in the same place every time. A cold shut would be a small air pocket or inclusion in the steel, which keeps it from reaching the right temp in that area and/or cools it down too fast in a small area. Often the difference causes fractures there just from the heat treat--then the slamming of the slide would crack it along the fractured area. Same thing could happen if the slides are forged. Stainless would be more susceptible because it's less tolerant of temperature variations. The same thing could be happening in the carbon steel slides and just not affecting them if it's small enough.