How do we get rid of this artefact?

In the forum page I only have [forum] shortcode (no break after).
But there is a pale box size of 1 uppercase character, on the next line after the forum.
When I inspect it, it is for the code block:

Code: https://pasteboard.co/U1EZcxbd40O9.png
Screenshot: https://pasteboard.co/xRgDyELLzay3.png