this doesn't sound like the passkey is the issue, is the implementation or the missing implementation issue.

in poland we have similar to passkey implementation for government profile, that is then used to login to most/all government websites or to sign government documents. you point the camera on the qrcode, confirm it on the phone and you are done. this same app has your ID, which can be used in most places (shops, banks, police etc).

and btw im using linux (main box), macos, android and ios - no issues so far with really cross device usage

This is the one Bluetooth feature that has always worked for me out of the box on both Windows and macOS. I haven't tried Linux yet to be honest, no idea if anyone has bothered implementing it yet. Usually open source is a couple years late to this sort of thing (except for maybe distros like Android or ChromeOS).

Now, if only Windows, macOS, and Linux can get together and fix whatever needs fixing to get headsets to connect properly automatically, that'd be grand.

I use this feature a bunch across dozens of different Windows and Mac devices from various device vendors with multiple Android phones and seemingly never had an issue.

But I'm also a person who generally never experiences the issues some people have with Bluetooth in general. If I ever have an issue with Bluetooth on a computer, I swap out the wireless chipset with an actually good one. Its almost always just bad hardware. I've only had to do that a few times in the last decade though, more modern WiFi/BT chipsets are generally pretty OK. Its the old ones that are near worthless.

Although I will say most of the time I just plug in my USB authenticator. I normally only fall back to the QR code if I don't have my keys on me.

And as an edit, I wasn't aware fully that the QR code is to help assist the BT handshake, I had assumed it was posting a signed request back to the service. My bad, my above comment isn't completely correct. Thanks for cluing me in to the BT requirement for the QR code path.