The issue is that villagers in 18w09a don't get willing to breed. They are not getting the green particles after trades and in any test set-up I did they wouldn't breed.

I've tested a few identical set-ups in both release and 18w09a and what works in release won't work in the snapshot.

I couldn't find anything anywhere that villagers changed for 1.13 content so I assume it's a bug.