OP here. Wow, yes, that was it! I put a "die" in there so I could study this one test while I was working on it. When I removed that, the tests worked as normal. And of course I didn't show the entire test suite in my example here, so I didn't include the "die" at the end of the relevant section.

Thank you so much! I never, ever would have figured that out. (In fact, it would have been even more confusing after I'd removed the "die" to run the complete test suite, and the test started working fine.)