The most likely cause is an inaccurate mask edge. If the black-and-white layer overlaps the subject slightly, it can create a bright halo along high-contrast edges like a hand or coat.

A few things can contribute:

• Mask bleed: the mask edge may be shifted or too hard.
• RGB editing: blending color and B&W layers in RGB can change luminosity at the transition, making edges look brighter.
• Sharpening/JPEG artifacts: compression noise and sharpening often exaggerate halos and edge contrast.

To fix it:

• Refine the mask so it follows the subject more precisely.
• Slightly feather the mask edge for a smoother transition.
• Check the edge at higher resolution, ideally from a non-JPEG source if possible.
• If your software allows it, try a luminosity-based workflow/color space to better preserve brightness relationships.

In short: clean up the mask first, then soften the edge a little and avoid letting JPEG/sharpening artifacts influence your edit.