This is one of those cases where things get bit blurry.
Why "must" she have known? Did it have a big red tag on it saying "Stolen by Nazis"? Having had three owners between Mauthner and Taylor makes a pretty good case for Liz not knowing. What action (if any) was taken against the first owner of record after it was stolen?
Suing for ownership because it was stolen property and suing because the owner three times removed should have "known" are two separate issues IMHO.
One of the reasons for the 1998 treaty was the embarassment in the western states over then trade in stolen artworks. Did Liz, or Sotheby's, know that a well know van Gogh which surfaced in 1963, having last been seen in 1939 Berlin owned by a Jew who fled Germany, might have a tainted title. If whe buys her jewelry out of the trunk of a car at the local Home Depot, maybe not, otherwise of course she knew the origin was suspect.