Having the same issue

The previous button needs to value a value attribute. In Forms.cshtml add value="1" to the prev button and that should fix it.

value = 1 solution with required fields on the second page, doesn't work

Same issue with Umbraco 8 and Forms - here is my fix:

    document.addEventListener("click", function (event) {
    if (event.target.name === "__prev") {
        var previousClickedElementName = "PreviousClicked";
        var previousClickedElement = document.getElementsByName(previousClickedElementName)[0];
        if (previousClickedElement !== null) {
            if (previousClickedElement.value === "") {
                event.preventDefault();
                previousClickedElement.value = "1";
                var form = previousClickedElement.parentNode;
                form.submit();
            }
        }
    }
}, false);

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