The bug

The merchant can be summoned with a negative age which has the effect that he has the hitbox of a baby villager.

How to reproduce

1. Activate hitboxes with F3+B
2. Spawn a Wandering Trader with an negative age:

   summon minecraft:wandering_trader ~ ~2 ~ {NoGravity:1b,Silent:1b,Invulnerable:1b,NoAI:1b,Age:-1000000}
   

3. Watch the hitbox of the mob

Example

Here is an example in video: The Wandering Trader can be summoned with a negative age.mp4