Whenever I try it "in the wild" (in production on a different device) it works. I just tried it again and it works. You can use an incognito window to test from-scratch ignoring any prior storage. I opened the latest release in an incognito window and waited for the "ready to use offline" notification. Then I went offline, reloaded the page, created a new project, and previewed it. It worked fine.
As with any issue report, it's usually impossible to make any progress unless you can find a repeatable series of steps to follow to reproduce a case where offline mode is not working.