Bug Report: Sony FX3 XAVC-HS Proxy Luminance Shift on Windows (Premiere Pro 25.6.2)

Summary of the problem:
When using Sony FX3 XAVC-HS (HEVC 4:2:2 10-bit) footage, QuickTime .mov H.264 proxies (using .mov to maintain audio channel parity) show a noticeable contrast/luminance shift when proxy mode is toggled on/off. This only happens on Windows. Running the same test on macOS with the same version of Premiere Pro produces a perfect match between original and proxy.

Steps to reproduce the bug:

1. Import Sony FX3 XAVC-HS 4:2:2 10-bit .MP4 footage (XAVC-HS).
2. Generate QuickTime H.264 proxies using standard ingest settings. 
3. Toggle proxy mode on/off in the Program Monitor.

What should have happened?

Original and proxy clips should look identical. Histogram and scopes should not change when proxies are enabled.

What actually happened?

On Windows, the proxy version appears slightly lower contrast. The histogram clearly shifts when switching between original and proxy.
The issue remains even when the clip is manually interpreted as Rec.709 before proxy creation.
The same workflow on macOS 25.6.2 produces matching proxies with no shift.

Camera metadata (from FX3 XML):

• Camera model: Sony ILME-FX3

• Codec: HEVC_3840_2160_M42210P@L51HT (XAVC-HS, 4:2:2, 10-bit)

• Gamma: S-Log3

• Gamut: S-Gamut3.Cine

• Matrix/Coding: Rec.709

This mixed metadata may be interpreted differently by the Windows HEVC pipeline.

Workaround:

Transcoding the FX3 clips to ProRes 422 LT before generating proxies produces perfect matches on both platforms, which points to a Windows-specific interpretation issue with Sony FX3 HEVC files.

Third-party plug-ins or panels installed:

(Installed but not used during testing)

• Artlist/Artlist FX packs

• Film Impact tools (now bundled with Premiere)

• Red Giant Universe

• Red Giant Magic Bullet Colorista V

• PluralEyes

System info (Windows system where issue occurs):

• Device: Dell Precision 7920 Rack

• Processor: Intel Xeon Gold 6246R @ 3.40 GHz

• RAM: 192 GB

• GPU: (If needed, add the GPU model here)

• OS: Windows 11 Pro for Workstations, Version 24H2

• Premiere Pro: 25.6.2

System info (macOS system where issue does not occur):

• Device: MacBook Pro 16-inch (2021)

• Chip: Apple M1 Max

• RAM: 64 GB

• OS: macOS Sequoia 15.6

• Premiere Pro: 25.6.2

Happy to provide a short sample clip and XML if needed.

Thanks,

-dp-