You're very welcome. I'm glad we figured it out.
If your website was a chapter in a book on a shelf in a library miles from your house, caching would be the photocopies of those pages in your desk drawer. The process of getting the book takes longer, has a lot of steps, and has more moving parts. Getting the photocopies is quicker and simpler with less effort. (That analogy fails when it comes to quality; photocopies are never as good as the original whereas cached copies are identical.)
A normal page revision or addition would have triggered the mechanism that updates the cache with the new version of the page. The only content on the testimonials page is the shortcode and since the shortcode didn't change, the mechanism wasn't triggered. I have not used WP Super Cache to know for sure but that's my guess.
The main goal of caching is to make your site load faster and if it does that then it's a smart choice. In my experience, sometimes it makes little difference and it's not worth the extra maintenance and unforeseen problems like the one you experienced today.
But your site was much faster with caching so I would recommend using it if
(1) you can configure it to exclude your testimonials page from the cache;
(2) figure out how to manually flush the cache (i.e. through the plugin admin or by deleting the cache) to force it to make fresh copies whenever you add a testimonial.
I hope that helps. WP Super Cache seems very robust and may have those abilities. In fact, you might try asking how to do that on their support forum.