When I updated my 1.14.4 survival world to 1.15, I found that my wool farm wasn't making any new wool; I found out that the sheep, despite eating the grass, would not convert it to dirt. Since my wool farm relies on an observer detecting said conversion, this stops the farm entirely from working.
I later checked on whether my villager breeder had the same problem, and sure enough it did. The villagers would not accept any bread I gave them, whether it was from the water stream (see below) or given directly in their spot.
After which, I attempted to revert the world back to 1.14.4. However, I found that upon doing this, all of the things that I had build were completely removed, as if I had created a brand new world, just with the same seed. Luckily, I had made a backup when I was prompted to, so nothing was lost.
(for some reason, the affects versions slot would only let me select 1.15.1, even though I encountered this problem in 1.15 first)
What I expected to happen was...:
1. The sheep to convert grass into dirt grazing.
2. The villagers to accept bread that I gave them.
What actually happened was...:
1. The grass remained as grass.
2. The villagers did not accept any food.
Steps to Reproduce:
Strangely, my attempts to recreate this bug have failed. I tried to test if the same thing would happen in my creative world, but it didn't. I figured that nothing went wrong because the world was made in early 2018, so the original version of the world was different. So I created a world in 1.14.4, spawned sheep and made sure they could graze properly. Then I closed the game and loaded the world in 1.15, but the issue did not reproduce. If it helps, my world seed is -222529955380484040.