User relations are not loaded when querying /users/{id} endpoint

[rokasaitenta]

Describe the bug
After I upgraded my Strapi project from 3.0.5 to 3.1.1, /users/{id} endpoint no longer populates the relational fields, i.e. my User model has a "has one" relation to some other collection and previously /users/{id} populated the relational field with all the data from the related item, but now only the item's id is returned. However, if I query /users?id=8 then the related fields are properly populated. All other collections seem to populate all the related fields properly.

Steps to reproduce the behavior

1. In Strapi's admin panel create a "Test" collection with a field "title" of type "text".
2. In Strapi's default Users collection add a new relational field with a name "test", a relation type "has one" and a related collection "Test".
3. Create a new item in the "Test" collection.
4. Create a user in "Users" collection and create a relation with the item created previously.
5. Now try to query the /users/{id} endpoint with the id of the user created previously.

Expected behavior
I expect the API to return a User object with a "test" field populated like "{ id: 1, title: "asdf", ... }", like it was in version 3.0.5.

Current behavior
The API does not populate the "test" field and returns only the id of the related item.

System

• Node.js version: 12
• Strapi version: 3.1.1
• Database: MySQL 8.0.21
• Operating system: Official Strapi Docker image "strapi/strapi:3.1.1-node12"

Additional context
My Strapi project is running in Docker container with official Strapi image "strapi/strapi:3.1.1-node12". It was version "strapi/strapi:3.0.5-node12" but I updated it because I needed OR and AND operators in REST API queries. I also followed the migration guide https://strapi.io/documentation/v3.x/migration-guide/migration-guide-3.0.x-to-3.1.x.html.