In test group "sg-and-defined-Sibling-3", the schema "wgData/sg/snbranch.xsd" is meant to test "interaction of substitution groups and #definedSibling".

The only substitution group in the schema is

 <xsd:element name="s1" type="xsd:string" substitutionGroup="tns:s"/>

But instance tests "snn1a" and "snn1c" seem to expect another substitution group:

 <xsd:element name="n1" type="xsd:string" substitutionGroup="tns:n"/>

If "n1" does not substitute "n", then it is not excluded by the "#definedSibling" wildcard, and the schema only allows an "a" to following "n1", which means "snn1a" is valid (but it's expected to be invalid) and "snn1c" is invalid (but expected to be valid).