I'd be very surprised if evidence obtained this way would be admissible at a trial. But if you want to be safe, use another finger for your Id card than for your phone...
For example, call recordings that were obtained without lawful permission can be thrown out and actually currently were in a highly visible case.
But I've mostly just got this from my wife complaining about it. There's very little structurally in place to prevent the police from abusing their power in germany, and when it comes to evidence collection, they are definitely incentivized to do so.