Dictation not working with external USB mic

I have a 2018 MacBook Pro that I often use in clamshell mode, attached to a Thunderbolt hub and an external display. When I'm using my computer in this configuration, I am unable to use dictation with my external mic. The mic is recognized in the Sound pane in System Preferences and works in most apps, including first-party/built-in apps (e.g., Quicktime) and third-party apps (e.g., Zoom, Teams, Skype). When I try to use dictation, however, the computer acts as if there is no sound input at all. Pressing fn twice displays the dictation microphone icon, which does not react at all to my voice. It never displays any dictated text on screen, even after I click "Done" or hit the fn key again.

Is there a workaround to enable dictation on an external mic? I saw that this was a "known issue" more than a year ago, and I was hoping there'd been a solution in the meantime.