Then take a look at the exported SVG file with missing the inner outline. It's also strange that the color is a gradient to brown on the outer stroke. See the image below...
Something happened on the inner side as you can see some faint anti-aliasing on the inner edge. Still a big fan of Pixelmator Pro - but hopefully SVG is will get some bug fixes soon. This was hit on Version 3.2.3 (Lumière).