Cohost PVP update: pinned messages will remain pinned at the top of the user's feed page until they're old enough to not be on that page anymore. So a pinned post will stay at the top of page 1 until it's relegated to page 2, then stay at the top of page 2 until it's relegated to 3, etc.
If there are multiple pinned posts, they stay in order of oldest to newest.