If you research forestry, wheelbarrow, or survival techniques in the dark age before completing the Chamber of Commerce, the free trader won’t be created after aging up. I experienced this playing as French, not sure if it also exists for JS. Researching these techs after the chamber has been built awards the traders as expected.
I saw a related post claiming if all 3 techs are researched, it only skips the first trader (so 3 tech researched, 2 traders produced). I haven’t tested this, since I usually only go for 1 dark age tech at most.