I'm not sure if this is a bug from previous version changes or intended as new search logic, but I see the same problem in ubbcentral.com forums, currently at 7.4.1.
Example: As you read this post, click on the Search pull-down, then click Advanced. Does it automatically select "Version 7 Installation Assistance"?
My Perspective: Testing as I write this post(current URL:http://www.ubbcentral.com/forums/ubbthreads.php/ubb/newpost/Board/10.html), it does default to the correct selection. However, when I am reading posts in any of the ubbcentral forums, the advanced link defaults to "All Forums" in the advanced search form.
Looking at scripts/search.inc.php, I'm guessing it has something to do with the preg_match using #Board without consideration for #forums after recent changes to URL structure?
I posted here before reporting as a bug. What am I missing?

It defaults to the top and ALL FORUMS for me. Just tried it here as you explained and did not get your results.

Tried a couple other forums and same results for me. Goes to the ALL FORUMS by default.

Thanks for checking ntdoc.
Looking at my current 7.1.1 production site, it does default to the current forum. My 7.3.1 test site does not. I'm guessing something was missed after 7.1.1, unless it's newly desired search behavior.