ProRes 4444 alpha is fully opaque on macOS; identical media works on Windows

Premiere on my Mac reads the alpha channel of a ProRes 4444 MOV as fully opaque. The Source Monitor's Alpha view is entirely white, and the clip covers the background with a black rectangle in the sequence. My colleague sees correct transparency from the identical media in Premiere on Windows.

Environment
- macOS 27.0.1, build 26A434; Apple Silicon (arm64).
- Premiere Pro 2026: installed version 26.5.2.
- Premiere Beta: installed version 27.1.0.
- The issue occurs in both standard and Beta Premiere.
- No proxies are used.

Steps to reproduce with the affected media
1. Import the affected ProRes 4444 MOV on the Mac.
2. Open the original clip in the Source Monitor.
3. Select Alpha display: the image is entirely white.
4. Place the clip above a background in a sequence: transparent regions appear opaque.
5. Compare with the identical source media in Premiere on Windows, where transparency displays correctly.

Expected result
The Source Monitor shows the actual alpha mask, including transparent and partially transparent regions, and the sequence composites correctly.

Troubleshooting
- Modify > Interpret Footage identifies Straight Alpha.
- Ignore Alpha Channel is unchecked.
- No proxies are attached or used.
- Media is identical on Mac and Windows, as checked by the users.
- Resetting preferences and clearing the media cache did not resolve the issue.
- Reproduction in a fresh empty project has not yet been independently confirmed.

Read-only analysis of the affected portal MOV on the same Mac
Using PyAV 15.1.0 / FFmpeg libraries, all 171 frames decoded successfully and all contained transparency. The original file was not rewritten.
- Codec: ProRes; profile: 4444.
- Decoded pixel format: yuva444p12le.
- 1920 x 1080, 30 fps.
- Encoder metadata: Lavc60.31.102 prores_ks; vendor: FFMP.
- Alpha values span 0 to 255 after conversion to 8-bit RGBA for measurement.
- At frame index 60, approximately 59.66% of pixels are fully transparent and 18.51% are partially transparent.
- Compositing that decoded frame over a checkerboard displays transparency correctly, including soft smoke edges.
These analysis results refer specifically to the tested portal MOV; other affected assets have not been analyzed.

Possibly related report, not a confirmed root cause:
https://github.com/CommandPost/FCPCafe/issues/359

Please investigate why Premiere on macOS reads this alpha as fully opaque while the identical file works on Windows and FFmpeg decodes transparency on the same Mac. Is there a supported decoder setting or fix that preserves the original media and shared project? Transcoding or changing the shared timeline is not practical for our Mac/Windows collaboration.

The source MOV and diagnostic stills can be supplied through an appropriate Adobe upload channel if required.