What you were expecting to happen, and what actually happened I expect topics that are “pinned” to be at the top of the forums, because they are important. Clearly, the topic about changes to bug reporting has a different plan and is not properly pinned.
How often does this happen? When did it begin happening? So far it looks unique to the topic.
Steps to make it happen again I’m not really sure because I cannot create pinned topics.
Discourse has a setting where you can have pinned topics unpin themselves for you when you’ve read to the bottom of them. So what has happened in @Slypenslyde’s case it’s likely that they’ve read the topic in the past, so it unpinned itself. (What I’m not 100% sure on is why the pin icon is pointing down, when unpinned ones are usually pointing up.)
This behavior can be changed by going to your profile settings (click on your profile image in the top right of the forums then the cog icon) and either checking or unchecking this box based on your desired functionality.
Actually dang, I’ve always been struck by how much Discourse beats the snot out of any other forum software I’ve used. Everything else feels so primitive.