Here's the explanation to the attached screenshots:
1. I created a simple rounded rectangle, 20x20 px, snapped to the pixel grid, with a corner radius of 2px and a stroke (aligned to inside) of 1px.
2. I expanded the stroke, which changed the nodes and made the rounded rectangle...round – but not just its corners.
I have to manually convert nodes to sharp to fix that (NOT shown in the image), which is rather tedious, and I'm pretty sure that's a bug.
3. Screenshot #3 is the same as #2, only zoomed in.
Is it a known problem? Thanks.