The 'Ensure JSON Schema' assertion is not throwing any error when mandatory fields are missing in the JSON request.
This assertion is failing only when request structure is incorrect. 

Release : 10.4, 10.5

Component : CA Service Virtualization

Entries in the schema that are mandatory can be configured with the required keyword.
Here is an example schema configuration and a payload that fails:

schema:

{ "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "number" }
 },
  "required": ["name", "age"]
}

payload:

{  "name": "john"
}

Assertion result:
Assertion name: Ensure JSON Schema checks for: true is of type: Ensure JSON Schema.

Test Summary
  JSON schema validation failed. Found 1 issue.
    Issue: object has missing required properties (["age"])
      level: error
      schema: {"loadingURI":"#","pointer":""}
      instance: {"pointer":""}
      domain: validation
      keyword: required
      required: ["age","name"]
      missing: ["age"]

For more detail regarding JSON schema's, review: 
https://json-schema.org/understanding-json-schema/