I first observed this while unlocking techs. Usually by the third tech unlock, a single researcher (starting with Learn 0) has gained several levels in learning (like to maybe level 3 or 4). This morning I started a new game and assigned a single researcher to unlock techs. By turn 17 about 6 techs were unlocked and the researcher was still only at a level 1. Hopefully this wasn't an intentional change So I browsed through all the diggers and wheel runners: there didn't seem to be any advancement on those skills either.
Start a game Assign a dup to researcher job Unlock several techs