Catalog import halts at exact photo thresholds (15,000 / 25,000) with "Unknown Error"

Lightroom Classic Version: 15.5

Operating System: macOS Tahoe 26.6.2 (25G83)

Steps to reproduce:

1. Open a primary Lightroom Classic catalog.

2. Go to File > Import from Another Catalog.

3. Select a large secondary catalog (e.g., 16,000+ or 43,000+ images).

4. Start the import process.

Expected result: The catalog import should process all images and complete successfully.

Actual result: The import process fails midway with a generic "An unknown error occurred" dialog box.

Key Pattern Identified: The import process stops at exact, round batch numbers:

• On a catalog containing 16,623 images, the import halted at exactly 15,000 images.

• On a catalog containing 43,881 images, the import halted at exactly 25,000 images.

Additional Diagnostic Details:

• Telemetry log right before crash: last_command = "ag.file.import.fromlibrary"

• RAM usage remains low (under 3 GB / ~8% memory total), ruling out memory saturation.

• The exact round numbers (15k / 25k) strongly suggest a batch transaction commit failure or an index buffer overflow during the SQLite database merge process.