This may be a browser rendering quirk? It’s an odd one.
I could only reproduce it in Chrome and the border itself is on top of a table row (which I don’t think can technically be shorter/taller across its width at various points, it’s all or nothing)… the issue persists if I removed various columns one at a time… and it also fixes itself if you load more topics, adjust the width of your browser window (below 1110px), or change something associated (like the border width) in the inspector…
So I moved the border from the first row to the top of the table itself, which avoids the problem and looks identical.