I tried Hymn and in my case works as expected.
Before the Hymn spell, I farmed 13 resources from one village, after Hymn I farmed 16 resources from one village which is correct since Hymn provides +20% resources.
Before the Hymn spell, I was able to trade 3000 silver for 3750 stone (ration 1:1.25), after the Hymn spell I was able to trade 3000 silver for 3750 (+ another 375) stone. That is also correct because the spell also provides +10% trade output.
Please notice that this spell does not affect production. It affects resources collected and traded from farming villages.
Can you please check once again and possibly confirm?