BuddyPress.org

What you could do as a temp measure is you couldstop the redirect happening by removing this elseif clause in buddypress/bp-code/bp-core-catchuri.php line 900:

} elseif ( bp_is_register_page() && 'register' == $front_page_component && is_user_logged_in() ) {

			/**
			 * Filters the logged in register page redirect URL.
			 *
			 * @since 1.5.1
			 *
			 * @param string $value URL to redirect logged in members to.
			 */
			$bp->canonical_stack['canonical_url'] = apply_filters( 'bp_loggedin_register_page_redirect_to', bp_get_members_directory_permalink() );
		}

Obviously this will be over-written when the plugin gets updated next.