I could be wrong but this sounds to me like one of the "flaws" in calibre's splitting logic during a conversion that I too have seen in virtually identical circumstances. I've never explicitly looked into it but it is my belief that calibre chooses to split when it sees an image before a heading tag, *regardless* of the fact that I have every split option turned off that I could find.
All I've been able to do in this circumstance is go back to the original and choose not to convert it in calibre, or else repair the damage in Sigil by merging each pair of pages together again (something that is dreadfully repetitive to do on a many chaptered book!).
Perhaps there is a secret setting I haven't found yet, or maybe it is just one of those hard-coded rules in the conversion pipeline. I'm sure Kovid or one of the other conversion gurus can comment...