Since upgrading my survival world to the pre-releases for 1.14.3 my automatic wheat farm managed by farmer villagers has produced nothing but bread. Whereas before the farmers were unable to pick up the wheat because their inventories were full of seeds, now they seem able to pick up the wheat and throw it to each other as bread.

This is a major change to previous behaviour of the same farm under 1.13 and earlier releases of 1.14 up to 1.14.2.

UPDATE 2019-06-08 (a):

Following the comment by @violine1101, I cannot be absolutely certain when the inventories of these farmers became corrupted. All I can say for sure is that the corruption only became a big issue when I upgraded to 1.14.3-pre-2. This world has been in existence since 1.13.0 and has been through all the major point releases since then, but 1.14.3-pre-2 is the first pre-release I have used for it.

There were some hiccups making the necessary changes migrating from 1.13.2 to 1.14.0, but the wheat farm continued to work after assassinating and replacing the old brown-coats who were not farmers, and of course installing composters. There was a small problem with the occasional bread getting collected, but it did not seem to get worse so I left it rather than exterminate all the farmers. The farm continued to function apparently normally through 1.14.1 and 1.14.2, but the upgrade to 1.14.3-pre-2 seems to have tipped some balance point.

UPDATE 2019-06-08 (b):

I have now made a small test world in 1.14.3-pre-2 containing a 2-villager wheat farm. It was a very 'standard' design with a hopper minecart track for collection, and two adjacent 9*9 farms with a waterlogged stair in the centre and a composter on top. Plenty of lighting was provided.

Initially I tried to 'feed' the farmers seeds before introducing them to the farm, but they refused to pick them up. I therefore started the farm with the villagers/farmers having empty inventories.

I increased the tick speed to accelerate crop growth, I don't think this changed the villagers' behaviour at all.

As soon as the crop matured the farmers initially harvested furiously. I noticed they picked up mostly wheat, but they re-planted so they must have picked up some seeds as well. However, after a while I noticed the farmers lost interest in farming. It seemed like they had now filled their inventories. They left the crops alone, and wandered around aimlessly.

During this time the minecart collected 4.5 stacks of seeds and less than half a stack of wheat.

I did not see any bread-throwing behaviour during the short duration of this test, but if this is the only way the farmers can make more space in their inventories I imagine they will now remain torpid until they have something to throw bread at.

UPDATE 2019-06-10:

I can now confirm the change in behaviour occurred with the 1.14.3-pre1 update.

I can also confirm that, when given another villager to feed, the wheat farmers throw bread.

There is therefore no longer any way to create a fully automatic wheat farm.

SUMMARY/TLDR:

Fully automatic wheat farms are now impossible to build.

In earlier versions I would have expected the farmers to avidly pick up seeds, which would have prevented them picking up so much (or indeed any) wheat. This seems to be the key difference, farmers are not interested in picking up seeds any more.