Wandering trader still spawn when doMobSpawning is false

Put the summary of the bug you're having here

What I expected to happen was...:
After I type /gamerule doMobSpawning false, No creatures will spawn

What actually happened was...:
Wandering traders and llamas still spawn

Steps to Reproduce:
1. Create a flat world
2. type /gamerule doMobSpawning false
3. But Wandering traders appeared without any spawn eggs or commands