The test method fails in the sandbox test framework because it relies on existing data in the sandbox, which may not be consistent or available for the test execution. The Apex test framework uses a separate test data context that is rolled back after the test execution, so any data that is queried or inserted by the test method must be created within the test method or by using the @testSetup annotation. The Execute Anonymous window, on the other hand, runs the Apex logic in the current user context and can access the existing data in the sandbox. Therefore, the Apex logic may succeed in the Execute Anonymous window but fail in the test framework. References:
* Apex Testing (Salesforce Developer Documentation)
* Isolation of Test Data from Organization Data in Unit Tests (Salesforce Developer Documentation)
* Free Salesforce Platform Developer 1 Practice Exam (With Answers) (Salesforce Ben)