I've recently started getting unusual artifacts whilst using Select Subject. By default I have it set to 'Cloud Processing' & when using this it creates a white border at the bottom of my image (on the mask). This is definitely a new scenario that I've not had before so I wonder if any recent changes might have caused this.
I changed my default processing to 'Device' & re-did my steps, this time I did not see the mask issue.
So I believe this suggests its local to Select Subject's Cloud processing option. I use this tool everyday on multiple images as I'm a e-commerce retoucher, it happens every single time without fail.
1. Adobe Photoshop 24.3.0
2. Ventura 13.2.1
3. Run select subject (cloud option), make mask. That simple.
4. Issue presented below. If the cutout meets the end of the bottom canvas (haven't tested left, right & top) it will leave a line of black.