OK... clearly, the extra start item (with no title) in the Guide section of the OPF is being introduced by MobiUnpack. It's not so much a bug as it is maybe a little too much -- thoroughness
-- let's say.
The bad news is that it has nothing to do with the issue at hand:
1) The issue affects KF8s built by Kindlegen from source code that has never been touched by MobiUnpack.
2) When re-building code created by MobiUnpack with Kindlegen, it clearly says in the output that it's ignoring the reference item with no title.
I'm of the opinion that it's a bug in the device firmware, myself.