The bug

/summon has a spawnEvent option to run a behavior event when spawning the mob. This does not work for villagers.

How to reproduce

/summon sheep ~ ~ ~ minecraft:entity_born

→  Baby sheep is spawned

/summon villager ~ ~ ~ minecraft:entity_born

→  Adult villager is spawned

The entity_born event does exist in the villager behavior file. This bug happens for all villager events, e.g. become_witch.

Notes

This is almost certainly caused by the fact that summoning a villager actually summons a "villager_v2", after some kind of transformation. This transformation is not seamless and causes various minor issues, of which this is one. The best fix is to remove villager_v2 entirely and have villager take its behaviors, because splitting the mobs like this is a horrible hack that should not exist.