It seems that if you have maxed your natural language slots (from intelligence) and gift of tongues, the bonus language slots from class, such as bard or monk, prevent you from gaining any progress from npc lessons, from Tinkers and other similar npcs.
I saw this happening to a monk character and now happened twice to my character (bard), after they maxed their "natural" language slots.
Class bonus language slots and Tinkers
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