I have a similar issue with sidebars and bbPress.
I installed bbPress and imported a phpBB forum with no real problems.
I used the default Sidebar2 and added login widgets etc to it.
I created a test page called forumstest and used the forum shortcode
When I directly use the URL for the forum page: http://www.weavervalleycc.org.uk/sidebar shows correctly. If directed to the forums with the built in ‘forums’ name, the sidebar defaults to Sidebar1.
Any ideas why this is and how I get around it?