That's a wonky symbol! For some reason, TWO rectangle were used to create the image placeholder. So I think the combination of those rectangles and the position of the 9-slice guides is causing the issue.
Quick fix is to open the symbol and delete the bottom most rectangle.
Or just create you own.
Thanks for getting looking into this, Jim.
I'm more curious than anything else.
What's odd is if you delete one of the line graphics in the symbol, the same result will happen - the stroke will no longer become 10 pixels in size. If you rename one of the line graphics, the same thing will occur.
It made me wonder if there was some sort of scripting going on, but I couldn't find any.
Of course I can make my own symbol, I am just really curious as to how this symbol works the way it does. I don't like working with magical tools or symbols, as it may lead to unexpected results down the road.
It's just a basic graphic symbol, so there is no scripting involved. Not sure why it is behaving in this manner, to be honest.