This person is the hero we needed: What is the empty space in GPU Visualizer? - #12 by ClockworkOcean
GPU work that isn’t explicitly instrumented will be included in a catch-all [unaccounted] stat. If that gets too high, it indicates that some additional SCOPED_GPU_STAT events are needed to account for the missing work.
from GPU Profiling | Unreal Engine 4.27 Documentation
It’s just disappeared from the latest profiling doc updates, or I missed it. 