On my square blog, it does not show the related posts. Nor does it show on your Square theme demo.
Does the Square theme not support Related Posts? Its setting is active in Publii with the Square theme.
If not, could you please please add it in the next Square theme update, thanks.
The Square theme even though your first or original theme is a theme of true beauty with the large side image on the desktop and the parallax kind of featured image on the mobile saving a lot of space on the top.
My site is at omheals.org. You can see how beautiful the Square theme looks. So please please add Related Posts to it.