Protestants are, of course, bound by divine not, but not by canon law. As they are not bound by canon law, their first marriages are presumed valid and even sacramental (assuming that both parties are baptized) even though canonical form is obviously lacking. As they are bound by divine law, second marriages are presumed invalid and must be convalidated if and when an annulment for the first marriage is granted (assuming a Protestant wishes to convert or is in a putative marriage with a Catholic).
That being said, as canon law does not apply here, couldn’t some second Protestant marriages technically be valid? Assume a Protestant party enters into a marriage that lacks certain essential elements - for example, one party was not open to life. This putative marriage ends in divorce. Later the same Protestant party enters into a second marriage but this time both parties are open to life and all other essential elements of marriage are present. As the Protestant party is not bound by canon law, under divine law alone, would not this second marriage be valid even though no tribunal process is ever invoked? This is obviously purely hypothetical as the Protestant in this scenario would not have any reason to seek an annulment from a Catholic tribunal.