On some maps the possible Trader locations will be at or near the max range which puts them out past (or mixed with) Plains biomes. Those maps *may* have a problem with the upcoming Mistlands update because in exploring that far to find a Trader you will cause the current temporary Mistlands biome to be generated - preventing it from being updated later.

If you haven't found it after doing a search from boat along nearby Black Forest shorelines, you might want to have someone verify your world seed's Trader locations using the World Generator web site.