Me too, Ive had males with a white spot on their bums, although its not an ovipositor, it could easily be mistaken for one. Ive also had males bar up and a female kill another betta.
You can get crazily masculine females with very long fins so short fins isnt an indicator either haha
Nearly all my wild species males have one. It's why it makes it hard to sex some of them because they can be nearly identical in appearance. I find the ovipositor is not a very good way to determine sex.