In the code it show the stroke to none but it should be visble in the web browsers. Polygon with border exports in svg to two paths - one for the fill shape and another for the border. I think thats the way Adobe XD team coded to export the polygon.
The border is not displayed because the exported path for the border is missing a fill.
It should be like this:
For me, it's working fine.
I have used XD version 18.104.22.168 on macOS 10.15.6 and Google Chrome to preview the SVG.
That's strange! I am also using 22.214.171.124 but on Win10. Can you also share the svg source code? Are other people experiencing the same on Win10