When a mob transforms into another (e.g. villager to zombie, or piglin to zombified piglin in 20w17a snapshot) it's Portal Cooldown NBT tag is reset. This can be seen in-game when a mob passes through a nether portal, then doesn't leave the portal on the otherside (e.g. due to blocks preventing it from doing so). Normally the entity's Portal Cooldown number would stay at 300 as it hasn't left the portal, however if that entity is then transformed into another (e.g. is cured, zombified, etc) it's cooldown is reset to 0 and it immediately teleports through the portal after transforming.

As the new entity is meant to represent the original having gone through a transformation, it seems logical that they should have the same portal cooldown, as there is no apparent gameplay reason for a newly transformed mob to bypass the normal cooldown period.

I also used repeat command blocks to display the entity's PortalCooldown in a scoreboard, and observed it reset to 0 after transforming (if still in the portal. it was only 0 briefly before returning to 300 as it passed through the portal; I also tried breaking the portal just before transformation and observed the counting down number jump from ~200 to 0 as it transformed).