Instant tracking currently requires the anchor point to remain in the camera view for position tracking to work - it’s not designed for large experiences that need significant user motion.
We’re making good progress on a full world tracking / SLAM (simultaneous localisation and mapping) solution that will work in WebAR and will build a much more complete map of the environment which will allow position tracking to continue even when the original anchor point is not in view. We’re hoping to have something to share in beta in the next couple of months.
The main “instant tracking” thread is here which includes some discussion of the current limitations, and is where I’ll post news on the availability of the full world tracking solution.
Hope that helps!