@eatgoddamnshit
It's more so that Maomao detective ability and vast knowledge solved their suddenly new problem:
Let's resume what clues Maomao had:
1. Cows, that were never used before are suddenly added to the paper making facility
2. paper quality suddenly dropped.
That's all for the clues.
Stopping there, we can deduce that the cows may have been responsible for the reduction of quality of the paper, the question is how.
Now enter Maomao's general knowledge:
1. Cows like to drink water a lot.
2. Paper making use glue which require water.
Stopping there, we can deduce that the cows are drinking the water used for making the glue and somehow it badly affect the quality, scientifically how does that happen tho?
Now enter Maomao's medecine knowledge:
1. Saliva will soften hardened materials.
Stopping there, we can deduce that the cows saliva, interact with the glue by softening it, rendering the paper quality very tearable and fragile.
So from all of this, what could have a village of paper makes have missed and didn't knew?
They certainly may had suspicion about the newly introduced cows to be the cause, but killing them would have resulted in a failed investment, not a solution.
The cow drinking water, it may seems obvious to our common knowledge, but at the time? It wouldn't be surprising for them to not know how much water cows wants to drink (you would be surprised, but my neighbours, a new farmer without formation, did the same with feed, resulting in the death of one animal already).
The saliva softening property? They would never know that.