turn the topic title into a sticky element so that it is always visible, and
adds a "Back to topic list" link below the topic title.
However, when you come back to a long topic (i.e. you arrive directly at the end of a long topic), Discourse don't add the beginning of the topic from the DOM, including the title. So the title is not there to be made sticky or to receive the "Back to topic list".
Only if you scroll up to the beginning of the topic will the topic title suddenly pops up and be displayed as sticky. Moreover, since the "Back to topic list" link is added when the topic route is first initialized, it's too late now and the link is not there.
In a balloon topic, Discpage is supposed to:
However, when you come back to a long topic (i.e. you arrive directly at the end of a long topic), Discourse don't add the beginning of the topic from the DOM, including the title. So the title is not there to be made sticky or to receive the "Back to topic list". Only if you scroll up to the beginning of the topic will the topic title suddenly pops up and be displayed as sticky. Moreover, since the "Back to topic list" link is added when the topic route is first initialized, it's too late now and the link is not there.