The additional project notes box currently does not process shortcodes, if any, inserted into it (did not anticipate it due to lack of space there).
Will the testimonial inserted into the main content itself work for you? See an example below –
https://www.livemeshthemes.com/extinct/portfolio/justo-euismod-blandit/
The above example does not utilize a shortcode but you can use the shortcode within the main content of the portfolio page. Hope that works for you.