The My Posts page is not meant to be a master list of everything you’ve ever done, but only a filtering of those things you’ve opted to pay attention to (either through participation or other means) that have changed since you last were last around. You can tell what is read or unread by the link colour.
If you really want to see everything you’ve posted within a category then search for #Scrivener @drmajorbob . You can get half of that by starting the search from the category page you’re interested in.
And it’s worth noting that if you want a master list of everything, that’s in the Activity tab of your profile, accessible from the avatar menu.
It does also look like if you access My Posts without any category filtering, that may be more comprehensive, and similar to viewing Activity in your profile.