Paypal refuses to offer generating a passkey on either Firefox or Chrome on Linux. Spoofing the UA to Firefox on Windows does work, though. Whatever they're doing, it's not normal. I'm surprise you run into it so often.

This is the first time I've seen this issue, very weird. Then again, I had to apply some kind of hack to enable TOTP back in the day (something about faking an RSA key fob), so I'm not surprised Paypal is stil shit.