After browsing a subforum, I usually click on the Mark forum read button. Usually, this gives me a box to confirm. I click on OK, then click on he Return button and I am back in the Main Forum.
Sporadically, when I click on the Mark forum read button, it does not give me the confirmatory box, it just marks all messages in the forum as read. Now, when I click on Return, nothing happens apart from what looks like a page refresh. If I click Return again, it will now usually return. (Or I can click on Home.)
It seems more likely to behave this way if there are just a few unread threads.
This happens when I am using my Android Tablet and Chrome. I have not noticed it on my Windows PC.
John