The bug

Wandering traders will be targeted by zombies in a 35 block radius. This is also the case if the wandering trader is invisible (by drinking an invisibility potion), which renders the invisibility effect pointless. (Or, rather, its only effect is hiding the wandering trader from the player)

To reproduce

This is how I reproduced this issue, to make sure that no external factors would influence my test:

1. Create a flatland world
2. Disable the gamerule doMobSpawning
3. Spawn a wandering trader
4. Make sure that wandering trader doesn't move (for example with NoAI)
5. Spawn a zombie (or husk) 35 blocks away from the trader
6. That zombie will start to target the trader
7. Spawn a zombie 36 blocks away from the trader
8. That zombie won't start to target the trader, unless he walks into that 35 block radius
9. Set time to midnight
10. Make sure that the wandering trader drinks its invisibility potion (NoAI needs to be set to false for this!)
11. Spawn the zombies like before when the trader was visible
12. Same result as above!

Video

https://youtu.be/hDESxghxm6U