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%.
Thank you for your report!
We're actually already tracking this issue in MC-158788, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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