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:
/tick freeze/summon minecraft:zombie ~ ~ ~/data modify entity @n[type=zombie] DrownedConversionTime set value 3/data get entity @n[type=minecraft:zombie] DrownedConversionTime✔ The result is 3
/data modify entity @n[type=zombie] DrownedConversionTime set value -1/data get entity @n[type=minecraft:zombie] DrownedConversionTime❌ The result is still 3, when it should be -1
Can confirm. This also happens with skeletons' StrayConversionTime field, which was originally reported in MC-230935. It does not happen with piglins' TimeInOverworld field. I've updated the title and description to make the scope and reproduction steps a bit clearer.