In 20w12a this issue was fixed: MC-175239

I guess related to that one still in 20w13b the spawn rate of zombie villagers is far too low.

Steps to reproduce:

• Create a new superflat world in creative mode
• Set time to night
• Fly in one direction and count all zombies and zombie villagers - I counted more than 300 zombies and 1 zombie villager. Before it was around 5%.