Interesting. So if the target fragment/id is not found, the file name portion of the href disappears upon splitting. Have to see if something could be done about that. Wouldn't think that would be desirable.
Thanks for the examples (and for your eyes, Doitsu)! I'd seen this effect crop up from time to time, but never really figured out under exactly which circumstances it happened.
Good to know it can be avoided by validating (EpubCheck) before splitting, but it really shouldn't be happening, in my opinion, regardless. That's a fairly destructive side-effect.