When the Get Class Defaults node contains a struct pin, breaking the struct causes the resulting pin to read "StructName_StructVarName". This causes the blueprint to fail to compile if the pin is used in the blueprint.
BP fails to compile because it is looking for "TestStruct Member Var 0" variable that doesn't exist
Blueprint recognizes pin is part of the struct variable rather than a variable of the blueprint itself
Head over to the existing Questions & Answers thread and let us know what's up.