tiff export is not b&w image

Is there an existing issue for this?

• I checked and did not find my issue in the already reported ones

Describe the bug

I have a black and white 16 bit negative from my scanner. I import it into dark table and set the metadata to black and white. I even go to the color calibration and select gray. I set the export image to tiff, 16 bit, high compression with predictor, and b&w as grayscale. The export tells me that it's not a b&w image.

Steps to reproduce

See the description.

Expected behavior

export my black and white negative as black and white

Logfile | Screenshot | Screencast

No response

Commit

No response

Where did you obtain darktable from?

darktable.org / GitHub release

darktable version

5.6.0

What OS are you using?

Mac

What is the version of your OS?

macos Tahoe

Describe your system

No response

Are you using OpenCL GPU in darktable?

No

If yes, what is the GPU card and driver?

No response

Please provide additional context if applicable. You can attach files too, but might need to rename to .txt or .zip

No response

No source files, tests, logs, or code entry points are named. First reproduce the TIFF export using the reported 16-bit grayscale settings in darktable 5.6.0 on macOS, then trace the export validation that rejects the image; done means the reported black-and-white negative exports as a grayscale TIFF without the error.