那么多重选择是否设置了默认值呢？这些选择是否只在更新/向数据库添加内容时使用呢？
我真的不知道你是想使用javascript还是PHP，但是如果你使用PHP，检查if isset($multiselectfield){}有意义吗？
或者更简单地说，如果它有默认值，则检查if ($multiselectfield != defaultvalue) {}。如果它不是默认值，则检查if ($multiselectfield != defaultvalue) {}，否则默认值一定是被忽略的。
只是我的想法，但问题的措辞不太恰当，无法给予你一个充分的答案。我只是尝试一下。

对于JS和JQuery用户的

我就是这样解决这个问题的。

/*
 * This variable will be used to track whether:
 * The selected values for the dropdown before opening it is same while closing or not.
 * Reason behind tracking?
 * If user just opened and closed the dropdown without selecting or deselecting the options,
 * the handler assigned to onDropdownHide event will be fired that is not expected.
 */
let monthValueBeforeDropdownShown;

// This function simply capture the selected options
// and put it in the variable
function captureValuesBeforeDropdownShown() {
    monthValueBeforeDropdownShown = $("#drop-down-id").val();
}

$("#drop-down-id").multiselect({
    onDropdownShow: captureValuesBeforeDropdownShown,
    onDropdownHide: exitMonthChangeHandler
});

function exitMonthChangeHandler() {
    // If no values are present (nothing selected) we'll terminate
    if(!$("#drop-down-id").val().length) return;

    // Comparing the dropdown values before opening and after closing.
    // If there is no change we'll terminate
    if (monthValueBeforeDropdownShown.toString() === $("#drop-down-id").val().toString()) return;

    // HERE YOU CAN WRITE YOUR AJAX OR WHATEVER LOGIC YOU WANT
}

PS：仍然有一些余量，以优化这个代码利用现代JS高阶函数。