I had similar problem with @us40489.
iPhone XR
Headset: plantronics Voyager 3200
When I try Drafts dictation with iPhone microphone, it works.
When I try Siri and iOS dictation through keyboard microphone mark with the headset, it also works.
But when I try Drafts dictation with the headset, it doesn’t work.
I tried it with English and Japanese. The situation is the same for both language.
When I talk twice slower than the first trial, somehow it catches some of the content.
For instance, when I talk to Drafts as “I’m trying dictation,” it catches as “I know.”
Something would intervene in between and my voice should have been delivered to it not clearly.
It makes me difficult to use Drafts, it would try to understand what I say though.