The issue is with the front-end pages.
The only external script that WP-Juicebox loads in the frontend pages is the 'juicebox.js' file (which internally loads the 'theme.css' file). This is exactly the same as when embedding a Juicebox gallery into any other web page and it should not prevent a page from scrolling.
Knowing that the only code on the frontend pages generated by WP-Juicebox is the 'juicebox.js' file and the JavaScript embedding code, the problem would appear to be a conflict between the core Juicebox files (rather than the WP-Juicebox code) and the Media Grid plugin.
However, debugging would be difficult as both the 'juicebox.js' and 'media-grid/js/frontend.js' files are packed and obfuscated.
Unfortunately, the only solution I can think of would be to isolate the Juicebox gallery from the other code on your page by creating a gallery by a method other than WP-Juicebox (for example using JuiceboxBuilder-Pro) and loading the gallery into an iframe as documented (as Option #2) here.
Sorry, I am not familiar with MP6, is this something I should know about?
Sorry, I was referring to a different WordPress plugin named Media Grid which requires the MP6 plugin and is used in the admin section only (hence my questions).