For anyone who may experience the same issue here, turns out the library pwacompat was causing issues with the authentication. I haven't dug into what part was causing issues, but removing it from the index.html fixed the issue.
mhartington
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Network settings within the devices are set to automatic, so nothing special there.
Safari's History and Website data has been cleared, and all experimental features have been reset to defaults.
Trying to get a network trace might be difficult as the authentication page is opened in a new tab and web inspector is not able to connect until it's already failed to load the page.
I will attempt to provide more details in Feedback Assistant.