I'm just shy of 30 years working for a manufacturing company, often doing manufacturing engineering. I'm trying to picture an manufacturing environment where something like that does not get noticed during assembly, and I just can't. It is virtually impossible for someone to have assembled that and not seen it - heck, you'd have to be asleep even to miss the corner of that transfer cover. In manufacturing there are no "one off's". For all those that made it out the door there were many more. They clearly have major problems in whatever facility made that.
*
I got to tour the Stihl plant once. I was shown the MS250 assembly line from beginning to end. I remember how fast some of the steps were, like how fast the piston got stabbed in the cylinder.
They said they rotate people around to different stations, so they are trained at pretty much any station. This is to prevent boredom and to insure the line is covered for when people are sick, or on vacations.
A stratified boot would be harder to detect than one leaking on the main carb boot, when the saw was fueled and started by the salesperson.
But still, quality control should have identified the boot issue sooner.
I've seen factory installed boots pulled out of flanges of Stihls too, and they didn't even have a protrusion fitting pushed inside of a boot.