Why is my uninstall webhook receiving incorrect shop value?

[jmattiace]

I have a webhook endpoint to handle the APP_UNINSTALLED event. Most of the time everything works well, however sometimes we receive the webhook with an incorrect `shop` value. In these cases, the `shop` value turns out to be the website domain.

Example

Merchant's website domain: test.com

Merchant's shop: test.myshopify.com

In the above example, our webhook endpoint received "www.test.com" instead of "test.myshopify.com". 

Why does this happen?