It used to be possible to unpin ( and re-pin ) something from the feed by just tapping on the pin icon. However, on mobile, this no longer happens. The animation happens, and the page refreshes, but after the refresh, the topic is still pinned ( or un-pinned ). It works as expected on the Desktop.
To unpin ( or re-pin ) something on mobile , I have to manually go into the thread to unpin it.
Happens on Safari, iOS 12. Don’t know about other mobile OS’.