Zombie villager doesn't transform into drowned

Zombie villager for long time doesn't transform into Drowned, while normal zombie transform into drowned

Step to reproduce :

1. Spawn zombie and zombie villager on 2 blocks high water or more
2. Wait for a minute
3. See what happened

 

Expexted Result : 

Zombie and zombie villager transform into drowned

 

Observed Result :

Zombie villager doesn't transform into drowned