When selecting an item with Enter key, the rawEvent propagation is not canceled and if a button with submit behavior is on the page, it is clicked.  https://www.screencast.com/t/sGVXL2baFj

Including this script to the page can be a temporary workaround: 
var _onEnterKeyOld = Telerik.Web.UI.RadComboBox.prototype._onEnterKey;
    Telerik.Web.UI.RadComboBox.prototype._onEnterKey = function (e) {
        _onEnterKeyOld.call(this,e);
        e.preventDefault()
    }