Selection Brush no longer works with Greyscale Image in Photoshop Beta 27.9.0 (and previous version)

Product: Photoshop Beta 27.9.0 (and previous version)
OS: Windows 11 Pro 25H2 26200.8655

If the image is in Greyscale Mode, choosing the Selection Brush shows a circular selection brush as expected, but holding down the mouse button doesn’t select anything - no pink overlay is shown, nothing is selected. Switching to the Marquee Tool confirms that nothing was actually selected (there is no selected area).  Converting the image to RGB and using Selection Brush works properly.  Convert it back to Greyscale and it’s broken again.

This has been working fine for years but broke about a week ago (at least one previous beta version). 

It still works correctly in Photoshop Mainline 27.8.0.