The principle behind this comes from Illustrator where you have a Fill and an Outline Color. If you have an Outline with a partial stroke you can still give it a Fill which leads to the result you posted. I am not sure though how this is reproduced in Figma but maybe it’s the same logic.
What format did you import as?
It is entirely possible to add a stroke to a line. I create icons in Illustrator and export as SVG.
I could see myself drawing a rounded corner square and them using pathfinder to remove that section. Then just exporting it to SVG. Probably sloppy, and I like to think I would not do that anymore.
But maybe that is one explanation?
It’s just an artefact of the SVG import. No way to reproduce directly within Figma.