System Information
Operating system: Windows 10 Pro 64 bits
Graphics card: 2x NVIDIA GTX Titan X

Blender Version
Broken: master 2.81a
Worked: Worked on blender 2.79b

Short description of error
When shadow catcher is activated in an object located in a collection with the holdout option, alpha is truncated.
(more visible when there is motion blur)

Exact steps for others to reproduce the error
Based on the default startup :

• set render engine to cycles
• set Film parameters to Transparent
• activate Motion Blur
• set world backgound color to white (better visibility)
• delete light
• create a plane, scale x2
• move plane to a new collection
• set plane to shadow catcher
• set collection 2 (called by default, with the plane) to holdout
• animate camera for have motion blur (more visible when there is motion blur)
• in viewport render, you will have that :
  
• render.

without motion blur :

with motion blur :

same render result with and without alpha on composite node :

.Blender here :