Holdout edge becomes thin when composite

The left one is the final render.(reference)
The right one is composite one(using holdout to mask next object)
I don’t know why the edge of the holdout becomes thin and transparent that I can see red background now…

My environtment :
Blender 2.82,
Cycles render,
blackman 1.5,
1000x1000 pixel render size(I only show region here)

My method :

  1. set holdout for next object and render
  2. save as .png 8bit (tried 16bit too)
  3. render other one
  4. composite them in photoshop
  • composite using object ID and compositor in blender is fine, without any gap.
    exporting as image makes it gap, I think.