When you summon a Zombie with the NBT Tag "DrownedConversionTime:1" it summons a Drowned Zombie, which is how it should work.
But when you summon a Zombie Villager or Zombie Pigmen with this NBT Tag it still summons a Drowned, every other mob this does not happen, including Husks (it summons a zombie because it goes to zombie than drowned) and Zombie Horses.
Zombie Villagers and Zombie Pigmen do not drown so it does not make sense why it does with commands.
Environment
Windows 10, Minecraft 1.13-pre6
Linked issues
duplicates 1
Comments 2
1. Zombie horses don't have that tag.
2. Caused by MC-127491
Highly possible that this is intended.