Bug confirmed on MCPE Version 18.104.22.168
Device: Samsung SM-G935F, Mali-T880, Android 7.0
Farmer villagers are not using up wheat they have in their inventory when they are making bread to share with other villagers.
I tested this with the help of another bug which made villager drop the item they have in their hidden inventory slot when killed.
I threw a stack of wheat(the amount of wheat does not matter) at a farmer villager. The farmer villager then started making bread and threw them at another villager and when I killed the farmer villager. He still had full stack of wheat however he threw 3 bread at the other villager thus he should have used up 9 wheat.
Another way to test this is to give the farmer villager a stack of wheat(amount of wheat does not matter) and separate him and another villager using an fence. The fence will prevent the bread from reaching other villager and fall to the ground. This means the farmer villager will keep producing bread and with a stack of wheat he should be able to make 21 bread however due to this bug, he can make infinite amount of bread. You can collect the bread and see how much he produces
https://www.youtube.com/watch?v=stmvqL7kLF4 Video proof