It's not just the WTO. Last week, a NAFTA Chapter 19 tribunal that was considering a Department of Commerce AD ruling found zeroing to be impermissible:
In sum, on the issue of the permissibility of zeroing, the Panel remands the matter back to Commerce to re-calculate Mittal’s dumping margins without zeroing.
There were a lot of factors they went through to reach this conclusion, so it's a bit hard to summarize. See pages 8-40.
There were also a couple dissents, one by two panelists (pages 69-80) and another separate one by one of the dissenting two (pages 80-85).
