Okay, changing the user agent was enough:
WebEngineView {
anchors.fill: parent
url: "http://web.whatsapp.com"
profile: webEngineProfile
WebEngineProfile {
id: webEngineProfile
httpUserAgent: "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.129 Safari/537.36"
}
}