I'm the author of a plugin called the SellFire Affiliate Store Builder. It embeds affiliate advertisements into posts and pages by replacing a short code.
One of my clients is using the GamePress theme and it seems that the theme is garbling the HTML replaced by my shortcode. Newlines in the HTML are being replaced by
tags. There are some other issues too with comment tags being HTML encoded.
I've verified that simply swapping out the theme on his blog resolves the issue.