It isn't mentioned on https://replicate.com/flux-kontext-apps/professional-headsho..., but on https://replicate.com/black-forest-labs/flux-kontext-pro under the "Prompting Best Practices" section is says this:

> Preserve Intentionally

> Specify what should stay the same: “while keeping the same facial features”

> Use “maintain the original composition” to preserve layout

> For background changes: “Change the background to a beach while keeping the person in the exact same position”

So while the marketing seems to paint a picture that it'll preserve things automatically, and kind of understand exactly what you want changed, it doesn't seem like that's the full truth. You need to instead be very specific about what you want to preserve.