After some experimantation I found that farmers will only creat bread whe they pick up 3 pices of wheat in one go. Also they will ONLY use THIS wheat for bread creation, ignoring everything in the inventory (yes even the same stack). Whenever a farmer picks up less the 3 wheats at once he will simply keep it forever.

Examples (for clarification):

• Farmer has 0 wheat and 0 bread and picks up 3 wheat --> he creates 1 bread (has now 1 bread  and 0 wheat)
• Farmer has 0 wheat and 1 bread and picks up 2 wheat --> he creates 0 bread (has now 1 bread and  2 wheat)
• Farmer has 2 wheat and 1 bread and picks up 2 wheat --> he creates 0 bread (has now 1 bread and 4 wheat)
• Farmer has 4 wheat and 1 bread and picks up 4 wheat --> he creates 1 bread (has now 2 bread and 5 wheat)
• Farmer has 5 wheat and 2 bread and picks up 3 wheat --> he creates 1 bread (has now 3 bread and 5 wheat)
• Farmer has 5 wheat and 3 bread and picks up 2 wheat --> he creates 0 bread (has now 3 bread and 7 wheat)
• Farmer has 7 wheat and 3 bread and picks up 10 wheat --> he creates 3 bread (has now 6 bread and 8 wheat)

Since farmers will rarely get a chance to pick up 3 wheat at once and will never use the wheat previously stored in their inventory, this will lead (at least in a wheat only scenario) to a builtup of wheat (and seads) until the inventory is full and the farmer permanently stops working.

I do not believe this is an intended behavior