WPT: PASS despite failed assertion

I noticed a couple of tests that pass even if one of their assertions fails.
To see what I mean, add an assert_true(false) to 
fetch/api/cors/cors-preflight-redirect.js, for one.
My browser logs the AssertionError, but testharness.js doesn't catch it, 
so the test passes despite the failed assertion.
Is this a known issue? Can testharness.js be improved so the failed 
assertion doesn't just vanish?
I think the issue can be addressed superficially in the tests 
themselves, but at a higher level, is there something testharness.js can 
do to avoid concealing the issue by swallowing the failed assertion?