Another Portfolio Menu Strange Effect

I made a portfolio with a custom post type, nothing fancy, just standard. Then I made a page template to display the portfolio items (posts) in 3 columns. Then I made a menu item pointing to this page (the portfolio page using the 3 column template layout). So when I go to it on the front-end, all looks good, the “Portfolio” menu item is highlighted because it’s “active”.

However, if I click on a portfolio item in the page to view it, it loads the full item post in a special single page file “single-portfolio.php” (which is what you do for custom post types instead of using the default single.php file). It loads the page and I see the full portfolio item, image, content, etc, etc. But then I noticed my “Blog” menu item is now showing as Active and not the Portfolio menu item.

What is causing this and how to fix it? Seems strange because the portfolio is not using the default blog category taxonomy of WP, it’s using a custom post type with a custom taxonomy category type.