From the ticket:
In a material instance using the attached master material if A is TRUE and B is FALSE I expect to be able to configure param C in the UI.
However param C is shown only when UseswitchParam is FALSE.
When UseswitchParam is TRUE the param is not shown but it's still used (output is equal to C value, not 1).
The problem may be in FMaterialEditorUtilities::GetStaticswitchExpressionValue: you treat UMaterialExpressionStaticswitchParameter as a UMaterialExpressionStaticBoolParameter while you should treat it as a Staticswitch with a BoolParameter connected to Value and so, you should have get it's value and then explore the true or false branch returning its value.
I attached a possibile fix for the issue.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-147298 in the post.