Seems when an expression output is swizzled (Typically via component mask) we lose the ability to track if the value is constant. This causes the IsPropertyUsed check to fail, potentially falsely activating overheads for features like PDO.
See https://udn.unrealengine.com/questions/334007/material-attribute-usage-not-properly-checked.html or create material shown in smallmtl.png. Difficult to test without a debugger attached.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-41594 in the post.