Are the isparent and ischild terms broken?
I'm trying to use them to filter Jay Naylor's work by removing any sketches which are parent or child posts. The query looks like this:
jay_naylor -( sketch isparent:true ) -( sketch ischild:true )
But this query seems to have also filtered some sketches which are not parent or child posts. From the first page of the query, we lose post #5871927, post #5627403, post #5558871, and post #5518281.
Anybody have some insight here?

those posts have child posts that are deleted, it seems. I guess those still effect the search query even though they don't apply the attribute to the thumbnail or anything else anymore.