When unparsing, if a complex parent element has a specified length, and the children element do not fill that space, then those bytes need to be skipped using the fillByte. This should happen in the SpecifiedLengthUnparserBase, but it was broken so was commented out. It was broken because skipping is bits similar to alignment in that you cannot do it until the absolute position is determined.

If you lookat OptionalWSPTermWithExplicitLength and OptionalWSPTermWithExplicitLength2, those test should round trip, but are currently disabled due to lack of this feature.