Sorry, after further testing i noticed that if the registration fields are disabled, then the registration process is ignoring them. I had to emulate the disable with "readonly" and some style:
<script type="text/javascript">
window.onload = function () {
const queryString = window.location.search;
const urlParams = new URLSearchParams(queryString);
if (urlParams.has('user.email'))
{
document.getElementById("email").readOnly = true;
document.getElementById("email").style.backgroundColor = "#1e73be";
document.getElementById("firstName").readOnly = true;
document.getElementById("firstName").style.backgroundColor = "#1e73be";
document.getElementById("lastName").readOnly = true;
document.getElementById("lastName").style.backgroundColor = "#1e73be";
}
};
</script>