Embedded data is attached to email invites through a code that identifies the contact. There aren't parameters for each embedded data field in the url.
Your issue is that you are sending a test email, which isn't associated with a specific contact in the contact list. You need to send a real invite to a real contact.

Thank Tom, that's great to know.
Is there any way to test a survey's data piping without having to send a "real invite to a real [fake] contact"? I feel like I must be missing something obvious because that really feels a little risky.
I know other (non-survey) platforms that can do data merges are able to give you a simulated live preview to test with. I just expected Qualtrics would do the same.

It isn't risky if you do it correctly. Add a 'test' embedded data field to your survey to flag test responses. Create a test contact list with test contact(s) flagged as test. Then send an invite to the test contact list.
If a test invite is used to complete the survey, you can easily find and delete the response by filtering on the test flag.