Unfortunately, the issue returned after a few hours of usage.
The phenomena, T4om observed (jumping brightness) also occured for me: Using the FP for navigation in my car at night, the brightness started to “jump” whenever the slightest brightness changes occur.
First idea was to create a task with tasker, which deletes the adaptive brightness settings every few hours. Due to a lack of knowledge, I discarded that idea.
Second guess: I am trying this (slightly old) app GitHub - Android Auto Brightness