I developed and tested on a Mac as well on both Safari and Chrome and I might've on Firefox too. Which browser are you on? I've noticed some weird stuff going on in other sites with Big Sur Safari so Chrome might be a better bet for now.
I've been using Safari. However, I have now tried Chrome and it works perfectly.
It might be an issue with serving files from file:/// vs http:// URLs. I’ll look into it! Glad to hear it’s working now!