Selection Brush tool doesn't work on grayscale pages

Hi ​@woni_0466,

Thank you for reporting this. I appreciate the clear description of the issue. I am aware of a similar issue introduced in Photoshop version 27.9 that affects the Selection Brush tool specifically on grayscale documents. The bug causes selection operations (including creating new selections) to malfunction in grayscale mode; for example, clicking the Subtract button performs an Add operation instead of a subtraction.

Here are a few things you can try as workarounds:

• Convert to RGB temporarily: If your workflow allows it, convert your document to RGB mode (Image > Mode > RGB Color), make your selection with the Selection Brush, then convert back to Grayscale. Your selection should be preserved through the mode change.

• Use Quick Mask as your workaround: Since you confirmed Quick Mask mode works correctly, you can paint your selection in Quick Mask mode (press Q) with a standard brush and exit Quick Mask to get your marching ants. This effectively mimics what the Selection Brush does under the hood.

• Test the latest beta and check if it’s working there for you or not. You can get the PS beta from the Creative Cloud desktop app > Apps > beta.

• Roll back to version 27.8: If this is blocking your work and you have access to previous versions via the Creative Cloud app (Preferences > "Show older versions"), version 27.8 does not have this regression.

A few questions to help us track this more precisely:

• Which exact Photoshop version are you running? (Help > About Photoshop / System Info). Is it 27.9.1?

• What bit depth are you working with: Grayscale 8-bit, 16-bit, or 32-bit?

• Does the issue also occur if you create a brand-new grayscale document (File > New > Grayscale), or only on existing files you're opening?

• Are you on macOS or Windows?

Looking forward to your response.

Best,
Anshul Saini