Twofactor_u2f now broken had to revert to totp

I’ve been using Nextcloud for years, and 2FA for about a year. This week on my gentoo LAMP box, with a new install of nextcloud-21.0.5, I found that I can install the twofactor_u2f app (which I have been using successfully with YubiKey for a year), but I cannot add new yubikeys as u2f devices. I had to revert to use of totp to keep twofactor enforced.

On my older nextcloud servers (some 21.0.5 and some 23.0.1), this week u2f stopped working, and I had to hack my way back in and revert to totp.

what’s up with this - why can’t I simply use u2f like before?