{!parent filters= is prone to pure negative query

From the linked email

"q":"{!parent tag=top filters=$childFq which=scope_s:parent
"childFq":[ "-sku_type_s:25"],

If I take out childFq=-sku_type_s:25, it works. If I change it to
childFq=sku_type_s:21, it works.

so far workaround is

 childFq=*:* -sku_type_s:25