Portfolio Category buttons not working correctly

Hi, I’ve installed your theme but have noticed this bug (even on your demo website). Let me explain:

On the homepage (and also portfolios page when you click ‘view more’), when you click on the categories (brand strategy, digital arts, etc) the thumbnails will rearrange and hide/show the corresponding portfolio. However, I notice when a portfolio post has multiple categories they will only show up on ONE category and not others.

For example: the second post in your demo (the one with the wooden building) is in 2 categories, photograph and web design. When you click on the web design button this post is shown with 2 others. However, when you click on photography this post is missing…

Looking at the categories each post should be in:
Digital arts should have 3 posts, however, clicking on the digital arts button only shows 2.
Photography should have 4 but only displays 2.

This bug/glitch also happens on my installation. Please can you let me know how to fix this so that the posts appear in ALL of its selected categories (not just one).

Many thanks!

The page I need help with: [log in to see the link]