I don’t know why you assume it’s not there if it’s part of native Android. Settings --> Battery & Charging --> Battery Saver. And in Battery usage you can see the warnings for apps that use up more power than usual. If you don’t see any, then it’s not happening.
Best to contact support, they handle issues based on the number of incoming tickets.
https://forum.fairphone.com/t/instant-fingerprint-unlock-is-the-default-and-no-way-to-disable-it/100616?u=uppercase