Blue horizontal artifacts appear after applying Lens Blur on Mac mini M4 – Lightroom Classic 15.5 / macOS Tahoe 26.5.2

Environment

• Mac mini M4
• 24 GB unified memory
• macOS Tahoe 26.5.2
• Lightroom Classic 15.5
• Apple Silicon / arm64
• Lightroom Classic catalog with images using AI masks, Remove and Lens Blur

Problem

Lightroom Classic produces random blue horizontal lines/artifacts on images after AI processing.

The artifacts are variable and may appear only several seconds after selecting the image in Develop. They can also be present in exported JPEG files.

The problem appears to be strongly related to Lens Blur.

Reproducible behavior

I can reproduce the problem with the following sequence:

1. Open an image containing AI masks and/or Remove edits.
2. Apply Lens Blur.
3. Lightroom starts AI processing in the background.
4. After some time, random blue horizontal artifacts appear in the image.
5. The artifacts can also appear in exported JPEG files.

The behavior is not simply a display-only problem because the artifacts can be present in the exported JPEG.

Important observations

With Graphics Processor disabled:

• Lens Blur ON → blue artifacts can still occur.
• Lens Blur OFF → artifacts disappear after using “Update AI Settings”.
• Lens Blur OFF + “Update AI Settings” → image becomes clean.
• Re-enabling Lens Blur → the blue artifacts return.

This makes Lens Blur a highly reproducible trigger.

There is also a GPU-related component:

• With GPU enabled, the artifacts tend to appear after a delay.
• With GPU disabled, the problem is much less likely to occur, but Lens Blur can still reproduce it.
• If a JPEG is exported before the artifacts appear, the exported JPEG can be clean.
• If the artifacts have already appeared, they can be included in the exported JPEG.

Suspected cause

This appears to be related to Lens Blur's AI/depth processing and/or its interaction with existing AI masks/Remove edits.

GPU/Metal processing may aggravate or accelerate the problem, but disabling the GPU does not completely eliminate the problem when Lens Blur is enabled.

Workaround

The current workaround is:

1. Disable Lens Blur.
2. Use “Update AI Settings”.
3. Verify that the artifacts disappear.
4. Leave Lens Blur disabled.

Disabling the Graphics Processor also appears to reduce the problem, but does not prevent it when Lens Blur is subsequently enabled.