I don't know which model of fingerprint reader is in the 6th gen, but on the 7th gen, enabling the beta firmware made it work perfectly.
$ fwupdmgr enable-remote lvfs-testing
If you temporarily enable, apply the Prometheus fingerprint updates then disable you won't need to worry about installing unstable firmware that will affect the system more widely.
Firmware metadata has not been updated for 30 days and may not be up to date.
Update now? (Requires internet connection) [y|N]: y
Fetching metadata https://cdn.fwupd.org/downloads/firmware-testing.xml.gz
Downloading… [***************************************] Less than one minute remaining…
Fetching signature https://cdn.fwupd.org/downloads/firmware-testing.xml.gz.asc
Failed to update metadata for lvfs-testing: '48A6D80E4538BAC2' is not a valid signature