I cannot say what is happening. Just that I built a book by importing an HTML file and split it at the cursor (Ctrl+Enter). When I went to validate it, it failed due to missing <title> information. I had to manually add
to each section.
But I think I may have a clue: when I open the HTML file in Sigil, there is no title element in it. So when I split it at the cursor, Sigil is just replicating what is already there and not adding the missing element.
In previous versions, when I imported an HTML file (it's my usual process: Word > Save as Web Page, Filtered > Open in Sigil), Sigil added the <title> element. It must have, because I have never before had a file fail validation due to this missing element. It's not something I was therefore even aware of.