Thank you for the analysis.
audioworklet-addmodule-resolution.https.html is failing on release_or_beta before any subtests have been created.
The harness then assumes the test is a single page test and creates an imaginary subtest name based on the title for the file.
This test is not using setup() to wrap the code that is failing. Wrapping in setup() changes the failure to a toplevel ERROR instead of a subtest FAIL, which the avoids the problem of unexpected subtest names. I've proposed this change in https://phabricator.services.mozilla.com/D46685.
In general, can a failure during set-up cause a different number of subtests to run? We often may not notice fallout from removing stale state because hopefully at least as many subtests would run on nightly as on beta, but the reverse seems quite possible.
It would seem reasonable that release_or_beta conditions would be considered stale only if there were data to support that, but addressing that may be less urgent with https://phabricator.services.mozilla.com/D46685.