For the record, this is a problem with Safari (latest), and not with Orion. It happens intermittently, and a page refresh often fixes the issue. I have no idea why the problem occurs.
To test, you can use any of the PDF.js links on my course website. For example:
https://appliedcryptography.page/slides/#2-6
I recently added a setTimeout that adds a delay between the window load event and the loading of the PDF.js iframe, but I'm not entirely sure that that fixes the issue.