After troubleshooting steps (clearing browser cache, restarting computer)

To narrow down things, I made a simple test.html file which only contained the embed code for one of the calendars embedded in our site. On the first co-workers computer it still didn't work. I asked the co-worker to login to her google account so I could see if she could access google calendars through the normal interface (suspecting a firewall issue perhaps). After she logged in, my test.html file worked fine. I also eliminated an internal network issue by browsing to our site using my mobile data

Further tests confirmed that users who were logged into the google ecosystem could see the calendars and those not logged in (including my own tests using Chrome's incognito mode) saw a large empty space where the calendar should have been.

The calendars in question are all set to "Make available to the public". I even tried isolating a single calendar this way and pasting a fresh embed code into my test.html file to see if something about the embed code has changed over time. When logged-in to google, the tests work; when run through incognito mode, they fail. I also tried checking and unchecking the "make available" option to circumvent any sort of changes that might have taken place on google's side with respect to the setting.

Has google recently changed something about how and where embedded calendars are allowed to display?

This doesn't just occur on my website. I checked out another company that has a website with an embedded google calendar on their site and it behaves the same way.