@KevinH and Diapdealer:
I was finally able to reproduce this issue:
1. Copy the following code to Section0001.xhtml in an empty epub:
2. Switch to Book View mode, enter a random string, e.g. XXX, before the heading text and press Enter. Repeat this several times.
3. Press CTRL+Enter or select Split At Cursor. This will generate the following code in the first split file:
Needless to say that this is a user input error and shows once again that editing text in Book View mode is a bad idea.