macOS 26 “Tahoe” screen recordings capture only left channel from multichannel audio interfaces

Since updating to macOS 15 (Sequoia / Tahoe), the native screen recording tool (Screenshot toolbar or QuickTime) no longer captures stereo audio correctly when using external multichannel audio interfaces.

Steps to reproduce:

1. Connect a Focusrite Scarlett 8i6 (3rd Gen) or Vocaster One via USB.
2. Start a screen recording via the macOS Screenshot toolbar (⌘ + Shift + 5) and choose the Scarlett as the microphone input.
3. Play a known stereo audio source through the Scarlett’s loopback inputs 1–2.
4. Stop and play back the screen recording.

Expected result:

Audio is captured in stereo (both left and right channels).

Actual result:

Audio is recorded only in the left channel (Input 1). The right channel is silent.

The signal appears to be summed or flattened to channel 1 before encoding.

Notes:

• This issue occurs only with macOS’s native screen recording API / QuickTime.
• Third-party apps such as OBS or Audition capture full stereo correctly from the same device.
• Focusrite Control confirms that loopback 1–2 contain stereo signal, and other software verifies stereo input.
• The same setup worked as expected in macOS 14 (Sonoma).