When the "ConversionTime" of the zombie villager is set to -1, it means that it's not being cured. Same thing with "DrownedConversionTime" for all kind of zombies when is set to -1, which is when they not being convert into drowned. Despite that you cant set the "DrownedConversionTime" and "ConversionTime" data to -1 or any other negative value using "/data" or "/summon".

Steps to reproduce:

1. /tick freeze

2. /summon minecraft:zombie ~ ~ ~

3. /data modify entity @n[type=zombie] DrownedConversionTime set value 3

4. /data get entity @n[type=minecraft:zombie] DrownedConversionTime

   The result is 3

5. /data modify entity @n[type=zombie] DrownedConversionTime set value -1

6. /data get entity @n[type=minecraft:zombie] DrownedConversionTime

   The result is still 3, when it should be -1