Photoshop 27.9.1 (Apple Silicon): 16-bit Gray Gamma 2.2 documents display severe posterization with GPU enabled; TIFF data and prints are correct

Title

Photoshop 27.9.1 (Apple Silicon): 16-bit Gray Gamma 2.2 documents display severe posterization with GPU enabled; TIFF data and prints are correct

System

Photoshop 27.9.1
macOS Tahoe 26.6.1
Mac Studio M2
Eizo 10-bit display
GPU enabled

Steps to reproduce

Create a new 16-bit Adobe RGB (1998) document.
Draw a smooth black-to-white gradient.
Convert to Gray Gamma 2.2 (or Image → Mode → Grayscale).
Observe severe tonal stepping in Photoshop.

Expected

The gradient should remain perfectly smooth.

Observed

The Convert to Profile preview is smooth.
After the conversion is committed, Photoshop displays strong posterization.
A screenshot captures the posterization.
Saving the 16-bit TIFF and opening it in macOS Preview shows a perfectly smooth gradient.
Printing the TIFF with PrintTool/QuadTone RIP produces a perfectly smooth print.
Converting the grayscale TIFF back to Adobe RGB in Photoshop also displays smoothly.
Disabling Use Graphics Processor completely eliminates the posterization.

Conclusion

This appears to be a GPU/Metal rendering issue affecting Photoshop's display of 16-bit grayscale documents. The underlying image data are correct.