Interesting and disappointingly. In this case I was testing it with play services, using the nextcloud app. I believe they are using something slightly custom from an open SDK, and suspect the outdoor was at their end.

It seems strange Google put fido2 into play services, but that's certainly what they seem to "need" to do to get things shipped, given the lack of prompt Android release updates (don't even start me on longevity...)

Still unfortunate every time this happens, as AOSP just loses more and more functionality.