It's an un-closed <pre> tag, possibly because the entire gemtext post was in a literal (```) block.
I fixed it in the original post.
The reason it (un-closed literal) was because the first time I added the closing sequence, it literally showed up in the text, so I got rid of it, having no idea it could affect subsequent responses.
I don't know why it was showing up, but when it was, the closing sequence was immediately after text. When I relocated the closings sequence to being on its own line, suddenly all was well in The Land of (the) Literal.
There is a glitch in the pubtrix!