OK
Clearing cache and cookies did not help. I've just moved a topic and this is what I see after the move
The opening post has disappeared. So it's not the topic that disappears but the opening post.
Also note that the url (Code Error while compiling - #2 by J-M-L) posted in this reply is not replaced by the by the page title.
And obviously entering the URL in the address bar results in an OOPS.
FireFox on Windows10.
Note: behaviour is intermittent.