Blender Boolean modifier: Difference behaves like union Attached is an example in which difference modifier behaves like union.
Is this is a bug (I think so)?
See this .blender example in which I tried to use boolean modifier difference (Child is subtracted from Parent so modifier is on Parent)
Behavior
In both, the expected behavior is difference, but actual behavior is union
Failed attempts
Before posting here I:

Checked that my objects were manifolds
Checked that normals are pointing outwards (Edit Mode -&gt; Select All -&gt; Ctrl N)
No shared faces between any two objects
Removing doubles (double vertices)
Converting to triangles

I have read extensively on StackExchange and other forums, and watched guides, including this very relevant guide.
Any advice would be much appreciated.
You are trying to use a simple plane as boolean operand here - which is by definition not manifold (a manifold surface must be closed, since all its edges have to have two and only two faces). Behavior of booleans in this case is never really ensured…

Also, your 3DView only shows objects enabled for rendering, and your 'ObjA' object is disabled for rendering, so it's completely hidden from 3DView.

Other than that, I see no issue with this .blend, if there are please be more specific.