Topic bookmark button doesn't offer an option to undo when pressed.
I've tested this on desktop(Chrome) and mobile (iOS); if I click the "Bookmark" button on a topic, I get to see "Bookmarked" but no way to undo this. The "Unbookmark" button for topics only shows up after a page reload.
For comments, this does work: the button changes to "Unbookmark" after "Bookmark" is clicked.
Edit: Similarly, the "Unbookmark" button does not change to "Bookmark" after the page is reloaded the first time; it requires another page reload to be able to re-bookmark the post.
Originally posted as an edit to this topic.