Can Kindle Previewer 2.8 read the K7 portion of a mobi that was built by Previewer/Kindlegen 2.7 (and split by kindleunpack)?
If yes: I'd suspect a subtle change to the way KF8/K7 combo files are being built has made it necessary to update/upgrade KindleUnpack's splitter feature.
If no: I'd suspect a Kindle Previewer bug.
Or, if you really think about it... why would Amazon technically even need
to include the ability for Kindle Previewer to read stand-alone K7 mobi files at all any more? It (and kindlegen) only produce combo files now, so why would previewing a K7-only mobi file be supported?
Just food for thought. There could be lots of reasons for this, but I suppose it wouldn't hurt me to get off my butt and actually download 2.8.