MP4 muxer in Premiere Beta 27.0.0 is broken

$ cat > /private/tmp/claude-501/-Users-alonsoreyesdiaz/c444add0-eb6c-42c3-ae9e-5648e0c70a06/scratchpad/adobe-bug-report.md <<'EOF'
**Summary**

In Premiere Pro (Beta) 27.0.0, exporting to Format: H.264 produces an MP4 containing only a video track. The audio track is silently omitted from the container — no error, no warning. Exporting the same sequence to Format: QuickTime works correctly. This started after the beta updated on 2026-08-26; the same project exported with audio before that update.

**Environment**

- Premiere Pro (Beta) 27.0.0 (also reproduced when queued to Media Encoder (Beta) 27.0.0)
- macOS 26.6.2 (build 25G83)
- MacBook Pro, Apple M3 Pro (11-core), 18 GB RAM
- Premiere Pro 2026 (26.3.2) also installed
- Sequence: 1080x1920, 24 fps, stereo master, 15 audio clips across 5 audio tracks, 22.75 s

**Steps to reproduce**

1. Open a sequence containing audio on multiple tracks.
2. Export with Format: H.264, Multiplexer: MP4, Stream Compatibility: Standard, Audio enabled, AAC / 48 kHz / Stereo / 320 kbps.
3. Export directly from Premiere, or queue to Media Encoder.
4. Inspect the resulting .mp4.

**Expected result**

An MP4 containing both a video track and an audio track.

**Actual result**

The MP4 contains a single `trak` atom with handler type `vide`. There is no `soun` track anywhere in the container. The file plays video with no audio in any player.

On some export attempts, Premiere also leaves an orphaned `.aac` file next to the output, named `<ProjectName><UUID>.aac`. Decoding that orphan shows the audio rendered perfectly: 22.78 s, 48 kHz stereo, peak −1.0 dBFS, RMS −16.7 dBFS, matching the video duration exactly. So the audio is being encoded correctly and then not muxed into the container.

**Ruled out**

- Multiplexer was already set to MP4, not None.
- Audio section enabled; codec AAC, 48 kHz, Stereo, 320 kbps.
- `ExportAudio` is `true` on all entries in the .prproj.
- No muted or soloed tracks; audio mixes down correctly (verified by the orphan .aac above).
- Not storage related — reproduced identically on an external exFAT volume and on the internal SSD.
- Not a disk space, permissions, or file lock issue.

**Workaround**

Export with Format: QuickTime instead. The resulting .mov correctly contains `vide` + `soun` + `tmcd` tracks. Converting that .mov to MP4 afterwards with an external tool produces a correct H.264/AAC file, which further confirms the source audio is fine and the fault is specifically in the MP4 muxing stage.

**Regression**

Premiere Pro (Beta) was updated on 2026-08-26 and the failure began the same day. The same project exported with audio prior to that update.
EOF
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