postman testing on 'response body contains string' does not work

To narrow down the problem, I change the response body into something as simple as one sentence as text/plain.

this is a test

However, the following very basic function still fails:

tests["Body matches string"] = responseBody.has("this");    

Ironically this test works:

tests["Body matches string"] = responseBody.has("t");    

I am using the latest postman v4.11.1. It looks like a fundamental defect for postman.