I think Agatha Christie may have changed her writing style for the Parker Pyne series because of more of an immersion in domesticity, as the time when "The Case of the Discontented Soldier" was written, was close to the time of her second marriage. Perhaps she also wanted to turn over a new leaf (as she did in the Mary Westmacott novels), but this time be recognised as its creator, doing so within her realm.
For some unfathomable reason, I have always enjoyed Evil Under the Sun. Perhaps it is because of the relish I get when seeing characters contorted and misrepresented in adaptations, or the sheer satisfaction of a (not entirely conventional) ingenious plot, which makes Poirot question the most basic and obvious of assumptions.
I agree, Tommy! I'd also, perhaps, like it if we had Miss Marple's involvement in The Adventure of Johnny Waverly, as an aunt of Mrs Waverly, perhaps...? Or a friend, former neighbour, something of the sort. I would have liked to see her views on the kidnapping on children and her insights into Tredwell and Miss Collins.