Wandering trader spawns in incorrect places

Updated description by [Mod] GoldenHelmet Oct. 7, 2020
Wandering traders do not seem to have any spawning limitations at all other than spawning in the overworld only. Code analysis provided in this comment and the one following.

Steps to reproduce

1. Build a large platform and cover it with blocks that prevent other land mobs from spawning, e.g. bedrock, glass, buttons, source water, flowing water.
2. Wait.

Expected result

Nothing spawns.

Actual result

Wandering traders and their llamas spawn.

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Original description
Hello,

I decided to start playing on survival on Bedrock (I'm a Java-fan) and created a new world. There is nothing special in the world: Desert and forest near spawn. But then came the Wandering Trader... Idk how.

The wandering trader should only be able to spawn in village gathering sites, but they appear to spawn anywhere in the world. This includes oceans and caves.