Im using the cheat so we can see the full decimal numbers
Base gather rate
1st upgrade 10% - all ok
2nd upgrade tooltip says 20% tech gives ~23%
3rd upgrade tooltip says 30% tech gives ~33%
4th and last upgrade tech gives tooltip says 35% tech gives ~38%
At imperial age this is almost an economic theory card worth of increase gather rate on coin of ~9%
This is what the gather rate should be
Its not a bug, its because the base rate for fields is actually 0.377
<rate type="deField" resource="Gold">0.377000</rate>
but at the start of the game there is a shadow tech that reduces gather rate by 10% so that it would be lower then it would be otherwises
they did something similar with rice paddies where the base rate of the rice paddies is 0.34 but its nerfed by a shadow tech but compensated by the rice paddy techs giving an extra 10%
so the rate is correct its just scaling off something different probably to account for walking time