I am using ASP.NET Webform (yeah, it is legacy code..) with <asp:UpdatePanel> and <AsyncPostBackTrigger>.
When a button click triggers a async postback, event.onsubmit from <form onsubmit="javascript:return WebForm_OnSubmit();"> will be called and return the validation result. However it is not returned once zone.js wraps the event handler, resulting the request not passing validation.

The source code here shows that the wrapper function doesn't return the result of
https://github.com/angular/zone.js/blob/master/lib/common/utils.ts#L89

const wrapFn = function(event) {
        let result;
        result = fn.apply(this, arguments);

        if (result != undefined && !result) event.preventDefault();
};

Not sure what it the purpose of this code not returning result. Thanks for taking a look.