Google chose to reduce the security rating of the fingerprint hardware in the Fairphone 3/3+ in Google certified Android, which is Android with preinstalled Google Apps and services, which is what still an overwhelming majority of Android users wants to have. Thus Fairphone OS is Google certified and has to follow suit.
LineageOS as a non-Google certified community effort Android OS agreed to not adopt this change upon request by the Fairphone 3/3+ LineageOS maintainers … Official LineageOS 20 for FP3/FP3+ - #91 by TeamB58