So yea its a legacy issue or feature (depending on ur perspective)
Ur villagers stop working after a while if you have tasked them to ur ally’s farm/mill/paddy etc
But today I watched a US player using his villagers on aztec farms 
tried the same for my asian villagers, my vills went to idle after a while
BUG? FEATURE ? or US bias ?
around 20 min. or so
CC: @Erchere
They get kicked off if your ally tasks their villagers to the farm. Maybe that’s what’s happening to you.
I tested that with AI, but the AI had empty mills/plantation, will have to test it out with friend. But if you are correct, my asian vills should be able to consistently gather from mills/farms throughout the game without any issues.
About this (and I dont wanna be monotheme), aztec wisewomen age up doesnt boost anything else but native farms. So if for any reason they begin with mills, haciendas, rices paddies or whatever they dont benefit from it.
So yea I tested with my friends the bug/feature is there My Indian vills stop working on mills after a time.
CC: @Ekdal1378 , @M00Z1LLA , @Erchere