After trying out the behavior of @test in Julia 1.5 more, I’m thinking that this “phantom error” behavior isn’t a result of the tested code throwing an error at all. Rather, when it tells me “there was an error during testing”, without telling me anything about the error, what it means is just that the test failed.
My confusion was due to the behavior of Julia 0.4.6 / the corresponding Juno, wherein I would get an explicit message in-line telling me that a test failed. But, EG, @test 1==2 now displays the “phantom error” behavior.
As for the error I was seeing when debugging, that must have been something that happened in my original code after the point where the test failed.