Select Sky produces an empty mask on a merged panorama DNG but works on equivalent TIFF

System

• Lightroom Classic 15.5.1
• macOS 26.6.2 (build 25G83)
• MacBook Air, Apple M4, 24 GB memory
• Apple M4 GPU, 10 cores; Metal supported

Steps to reproduce

1. Merge source images using Lightroom Classic’s Photo Merge → Panorama.
2. Open the resulting DNG, approximately 13,000 × 4,000 pixels, in Develop.
3. Create a mask using Select Sky.
4. Enable the overlay and try changing the mask’s adjustment sliders.

Actual result

Select Sky appears to run, but no mask overlay is visible and adjustments have no effect.

Expected result

Select Sky creates an editable mask covering the sky.

Comparative tests

• Linear-gradient masks work correctly on the same merged DNG.
• Select Sky works on the individual source images.
• Select Sky works in Lightroom Classic on a full-resolution TIFF version of the panorama.
• Photoshop’s Select Sky also succeeds on the panorama.

These results suggest an issue with Select Sky processing this merged DNG, rather than a general masking failure or image dimensions alone.

Workaround

Export the merged panorama as a full-resolution TIFF and apply Select Sky to the TIFF in Lightroom Classic.