Chris: It could be that your cache/cookies are showing what it recognizes based on what you've filled out. The best way to test this out would be to keep clearing your cache/cookies so that nothing is stored.
Clear the cache/cookies and visit the landing page with the form directly to ensure that no pre-population is ocurring. Then, click on the shared link to the landing page to display the pre-filled information.
Amber: Unfortunately I won't be able to speak to the custom JavaScript that was added. However, in regards to how Marketo tracking works, anytime a link is being shared, it will be pre-populated with that lead's informaiton. Considering there is some third party software or custom code, I'd recommend testing it out.