What I expected to happen was...:
I expected villagers to breed as they have done in previous versions

What actually happened was...:
Nothing happened. They tossed food to each other but no hearts or babies

Steps to Reproduce:
1. Create a new flat world in 1.13 creative
2. Spawn 2 villagers and make 9 valid doors
3. Wait

I did find that if I created a new world in 1.13 than closed and reopened it in 1.12.2 they bred. Then when I closed it and re opened it in 1.13 they continued to breed.