Thanks gdenby, at least that's great insight.
Seems to me Designer is built to cater more toward the visual display of artwork than as a precise vector tool. The user shouldn't have to go through all those steps to clean up artifacts that the program generates (that's more work, not a benefit). Expanding a stroke should render the stroke with no distortion. At least that's what I've experienced in other vector tools.
Interestingly, I discovered this when I ran this file through a CNC Paper cutter (Silhouette Cameo) and the cutting head spent about 15 seconds cutting in that corner. It was following all those nodes! Even if you don't see these nodes, there is still something broken with the stroke expanding engine.
I'm super-impressed with the visual rendering of vector/raster artwork with Designer, but simple technical aspects like this that slow down my workflow are cause for concern.
I really want to be a fan, so I'm hoping this is somewhere in the backlog of bug fixes!