I do not know if it is a feature or a bug, but it is possible to train native units from trade route’s trading posts using hotkeys, while no UI button is shown.
Version 11148, still present
Windows 10, Steam
Map, Winter Wonderland
Selected two tradeposts by doubleclicking (or shift-clicking). One on trade route, one on native settlement. Instructed settlement TP to train Coureur de Bois. Frequency 100%. Screenshot. Can’t think of more information to add.