I was walking around my village and noticed a lot of yellow on the villager small wheat farm, And what caused it? The villagers not harvesting crops! How to reproduce? Good question, I think you will need a bunch of farmers to see if any of them will refuse to harvest crops.
(Note: Mob Griefing is ON!)

If you cannot reproduce I can give out a world download to the specific affected world, So you can see the problem more carefully! (Give me instructions on where to post the world download and I will give you the link, The coords to the village are in the screen during the video)

Videos and Screenshots below!

(Please dont mind the lag, It was only on the videos for some reason)

New:
I have done some experiments, Take a look at results:

1-Farmers DO re-plant, They dont just harvest them just as you can see here: Image
(That is way more grown than one of the images below)

2-If I clone the world in creative mode they work normally (?!?!?!?!)

Image of the villagers from a good distance

Video 1

Video 2

This issue was fixed for Java Edition in Snapshot 19w12a MC-145857

Steps to reproduce:

1. Open the attached world MCPE-47166 Harvesting demo.mcworld. This is a flat world with 2 villagers, a farmer and a butcher. The butcher's inventory is empty. The farmer has 4 bread but is otherwise empty. The world has 8 crops, 2 each of beetroot, carrots, potatoes, and wheat. The daylight cycle is disabled and the time is 5000.
2. Set the random tick speed to 100, or just wait for the crops to mature. Notice that the farmer wanders away from its job site occasionally, but never harvests any of the crops.
3. Exit the world and use a world editor to reduce the number of bread in the farmer's first slot to less than 3.
4. Open the world again. Notice that the farmer now harvests crops.

Update 7 July:
Farmers will also stop harvesting when they have 12 carrots or 12 potatoes. Since this isn't enough for them to share and they have no way to get any more, the only way for them to resume producing and distributing food is for a player to supplement their inventory, allowing them to share and reduce their inventory below 12. They will then harvest again, but will immediately get back into the same state, so it's not even useful as a workaround.