QuickTime screen recordings always mono with external mic in aggregate input

QT screen recordings are all mono. I'm doing screen recordings of a music track in a DAW (stereo) and an external mic (mono) via a Clarett Pre USB. I've set the correct aggregate inputs/outputs with Blackhole in Audio/MIDI yet all the screen recordings are mono.

When I only use Blackhole (ie. computer audio—the DAW output) I get stereo recordings. When I add the external mic, it's back to mono.

I'm on a 2018 Mac Mini. Every video I've seen suggests that I've got the right settings.