Guys, I can't get Sigil to create a uuid manifest id (valid or otherwise) no matter how I split a file: at markers; at cursor (in Book View or Code View).
EDIT: never mind... I see it. It only seems to happen when there's only one manifested xhtml file in the epub. If more than one file exists, the new split file gets assigned the generated unique file name as its manifest id.
I almost never work with one-file epubs. Easy to see how this would escape detection. Especially if no one ever reports it.
In the meantime: add a blank html file, do your splits, and then delete the blank html file to work around the issue until such time as it gets resolved.