The update properties of my DataCards had to be a little more detailled :

Role dropdown

{
    '@odata.type': "#Microsoft.Azure.Connectors.SharePoint.SPListExpandedReference";
    Id: Dropdown2.Selected.ID
}

Utilisateur combobox

{
    '@odata.type': "#Microsoft.Azure.Connectors.SharePoint.SPListExpandedUser";
    Claims: "i:0#.f|membership|" & ComboBox1.Selected.Mail;
    DisplayName: ComboBox1.Selected.DisplayName;
    Email: ComboBox1.Selected.Mail
}