Upgrading the Economic wing with Abbasid and getting the Imperial upgrade Improved processing, will allow your villagers to drop off twice the amount of stone that they should.
They can carry 15 stone, Meaning the Improved Processing Tech should make the Villagers drop of 16 stone each time. But currently they drop off 30 stone.
This bug does not affect Gold, food or wood gather.
Make Economic wing.
Go to Imperial Age.
Get Improved Processing.
Villagers now gather x2 stone.