Custom html page loads as widget, not as its own home page

I am working on implementing custom dashboards, including JavaScript and CSS etc.

I have a custom .html page, which is loading properly if I add it as a webpage widget. However, when I set a role's default page to the html file itself, it isn't loading, like the JS isn't loading.

I am using a custom .JS codepage referenced in the html page if that matters.

Any ideas what could be the cause of this? Has anyone come across this before?