Lightroom Desktop 9.5.1 on macOS: Square Grid Camera Raw preview memory leak peaks at 45.6 GB and thrashes swap

Incident date: 21 September 2026. This was measured during a production editing session, not a synthetic stress test.

Product and environment:
- Adobe Lightroom Desktop (cloud-based) 9.5.1 arm64 [20260815-1945-84a45bf]
- macOS 27.0 (26A428)
- Apple M3 Pro with 18 GB unified memory
- Canon CR3 originals stored on an external SSD
- Approximately 134 GB free on the internal drive and 65 GB free on the external drive
- No macOS thermal or performance warning was recorded

Summary:
While viewing a filtered set of edited CR3 files in Square Grid, Lightroom’s Camera Raw preview-rendering pipeline accumulated tens of gigabytes of memory and eventually made both Lightroom and macOS nearly unusable. Relaunching Lightroom immediately returned memory usage to normal with the same files and hardware.

Steps observed:
1. Open a local folder containing approximately 9,064 CR3 files.
2. Apply the Blue color-label filter, leaving 417 photos visible and 8,647 filtered out.
3. View and scroll through the edited RAW thumbnails in Square Grid.
4. Lightroom progressively becomes extremely slow and alternates between running and stuck states.

Measured behavior:
- Lightroom CPU usage repeatedly reached 114–392%, with a measured spike of 755.7%.
- Physical memory footprint reached 33.3 GB, with a peak of 45.6 GB.
- System swap usage reached 30.5 GB out of 31 GB.
- vmmap showed approximately 28.4 GB allocated in Lightroom’s default malloc zone, with 27.3 GB swapped out.
- The process sample showed multiple NuPreviewProviderTasksWorker.lua queues and Camera Raw threads named InterpolateBayer, RenderIncremental_B, and GPUImageView background queue.
- The SQLite page cache was only approximately 58 MB, so the catalog database was not the source of the memory consumption.
- The Assisted Culling database did not change during the incident, indicating that active Assisted Culling analysis was not the cause.
- Lightroom logs also recorded “AgCachedCreateAutoMasks exception: dng_error_unknown” and repeated “attempt to compare nil with number” messages shortly before the incident.

After relaunching Lightroom:
- Memory usage returned to approximately 1.3–2.1 GB.
- CPU usage returned to approximately 2–3% while viewing the same files.
- The system began recovering swap space.
- Cloud upload could continue at low CPU usage, so synchronization alone does not appear to be the cause.

Expected result:
Camera Raw preview-rendering tasks should release their working memory after thumbnails are rendered, and Square Grid should remain responsive.

Actual result:
Memory appears to remain retained across preview-rendering tasks until Lightroom consumes more memory than the available physical RAM, causing severe swap thrashing and making the application unusable.

This appears consistent with a memory-retention or regression issue in Lightroom Desktop 9.5.1 on Apple Silicon. I have a process sample and vmmap summary available if Adobe engineering needs them, but I have not attached the raw diagnostic files because they contain local file paths.