The issue is somewhere in your Qualtrics-configuration, not Inquisit. The random string (e.g. "R_2YDIWgiEuDWO1M8") is Qualtrics' participant identifier and that is what your survey forwards to Inquisit in a URL parameter called "subjectid". It does not forward a random numerical value. I ran through your survey and this was the result:
Inquisit reads the value of that parameter and uses/records it as its subject ID as it should.