Minimal Download Strategy – A technique that delivers a faster and more fluid page navigation experience, in pages and site templates that support it, by downloading and rendering only those portions of a page that are changing.
I inherited close to a dozen SharePoint sites as part of my responsibilities for Network Engineering. Several of these sites are using custom forms that follow the guide lines of Mark Rackley’s Hillbilly forms ( see link below). As I learned how to implement and modify this technique, I ran into issues in my test environment. It seems that the JavaScript code attached to the Content Editor Web Part in the lists default forms was not executing correctly. In order to get the forms to work I needed to disable Minimal Download Strategy in the sites features.
The Hillbilly Forms Technique uses the Content Editor Web Part to link to an HTML file that defines the form. I can only guess that the issue with the Minimal Download Strategy is that any links to code outside of the form are not getting loaded into the browser. By disabling the feature the browser is able to load the default form as well as the code linked to the Site Assets library.
http://www.markrackley.net/2017/05/04/customize-your-sharepoint-classic-forms-without-infopath/