According to a 2019 write-up on Bruegel (by a German author)
The US complains that the Appellate Body goes too far in its interpretation of the WTO treaties, in particular in the area of antidumping. This debate is of importance because the US has lost more than 90% of cases that concerned its antidumping legislation and their application.
While not giving the 90% loss figure, a CRS report (R46296) confirms the target of the US criticism
Antidumping laws and policies have also been at the center of dozens of trade disputes between the United States and its trading partners in the WTO. Reports issued by the WTO’s Appellate Body (AB) on the subject have been one of the primary targets of the U.S. Trade Representative’s criticisms of the AB mechanism in the broader WTO dispute settlement system.
That latter report also claims that
The [US] Antidumping Act of 1921 became the textual basis for Article VI of the General Agreement
on Tariffs and Trade (GATT) in 1947, the multilateral trade agreement that established the post-
World War II rules-based trading system and which was later incorporated into the World Trade
Organization (WTO) agreements. As such, the U.S. model of antidumping has become the
Which makes it more (even) more confusing why the US is complaining about how the WTO AB is applying something that seems to be (ultimately) modelled after US law.
So, which specific WTO cases form the core of the US concern that regard, i.e. where the US says the WTO Appellate Body overreached in its interpretation of anti-dumping measures?