Your the second person to say this, and I can't understand why it used to work and now is broken. Going back in a browser doesn't normally do much with a server, and I can't ever remember seeing this behavior. Knowing what browser you use might help, but I honestly have to say I don't see this one coming back.
The old board behaved like that for as long as I can remember in various versions of Firefox and IE.
That sort of behaviour is controlled by the HTTP headers and meta tags that the server sends with a page to indicate how proxy servers and browsers should handle caching.
Although given that going back goes to exactly the same URL as the back to index link I am surprised that the browser is handling caching differently for the two cases.
I've already got used to clicking the return to index link rather than going back.