I keep saying it, this is related to the mic inside the bud failing. From either overexertion, defect in quality, or some other factor yet to be identified. Hope that helps
That doesnt quite explain it, because:
1) Fit test only uses internal mic (Listens for sound coming back from ear canal)
2) If internal mic has failed, fit test CANNOT pass, ever.
3) If fit test can pass in any way, internal mic is still functioning.
My AirPods FAIL fit test when placed in ears normally. They PASS fit test if extra pressure is applied, or if tested outside of ears with finger over eartip hole. So the internal mic definitively has not failed.
EDIT: This is not to say that some people's internal mics aren't totally dead, but this can't be the universal root of the issue