XPath exists/not exists operator with child axis

There seems to be an issue regarding the Exists / Not exists XPath operators.

Given the following test contents :

+ nodeA

• prop1 = 'value'
  + child
• subprop1 = 'something'

+ nodeB

• prop1 = 'value'
  + child

+ nodeC

• prop1 = 'value'

The query //*[child/@subprop1)] returns nodeA.

The query //*[child/not(@subprop1)] returns nodeB.

The query //*[not(child/@subprop1)] returns only nodeB, it should return nodeC as well.