Natori has never given Diana the impression that she likes her (romantically). That's always been entirely from Diana's perspective alone. Diana is a good friend though and Natori's Oshi. Natori has treated her like a sister or fan. That's why this whole poly thing has never worked. Natori is not feeling romance for two people, only for Lapis. Now, Allie is here and we see what a person who actually feels romance for Diana looks and acts like.
I've made a bunch of comments in previous chapters discussing this throughout the series, so this isn't a recent thing. The story has always made it clear that Diana has "Diana Vision." Pretty much any "moments" she had with Natori were friendly from Natori's perspective, while Diana's perspective would be filled with romance cues. With Lapis however, the romance would come from both of them together. It was mutual.