I experience it as well. All the software I have tried, none of them have independent eyebrow tracking. I reached out to some devs of those softwares Unreal LiveLink, iFacialMotion 3DFaceMotion, VBridger, Vtube Studio, Waidayo to name a few and they all say it is a problem with the arkit. Some of them even send bug reports. This was around 1 year ago already. I would love to see this fixed, as it would widen the usage of great tracking from the iPhone. All I was able to find out is, that this issue started with iOS 12. I would love more people to point this out, to give this some more attention. Since the movement of the eyebrows are one of the most important body parts to make a variety of expressions possible.