_jis_It could very well be a hardware problem on our end. I have no complaints about adaptive brightness in good lighting conditions, it adjusts well and reacts quickly. My only issue is in the special case when ambient brightness is reported somewhere between 0 and 7 lux (or whatever unit of measurement is used), in my apartment that is the usual case after sunset and therefore this becomes a daily issue for me.
The ZF6 is my first phone where adaptive brightness works smoothly under any lighting conditions (inside at home/work, outside in shadow/sunlight) and I never need to manually adjust brightness. I have to say that on Android 10, my satisfaction with adaptive brightness has even increased compared to the A9 even though I was already satisfied with it on A9 compared to my previous phones. So I conclude that you two probably must have some hardware problem. Did you search the forum here to see if any other has already complained about adaptive brightness? I do not remember anyone having a similar problem.