Why is the computed property not overridden?

Question

Vue.js

There is a computed property

formattedDate() {
            let date = this.date ? this.date : this.value;
            return date ? moment(date).format(this.getFormat) : date;
        }

date is a property in data, when it changes, formattedDate is not reset, although the date property itself changes
https://codesandbox.io/s/vue-template-mqyo8
when you select a date the second time, the property is not reset. The property itself in components/date-picker/index.vue
UPD if you close/open the calendar (double click on the input), then the date can be selected again, but again only once

Answer 1

In fact, it is very strange that it does not work either when moving from a computed property to a method, or when using this.$set. And forceUpdate did not help.
The only way to fix it is to create a formattedDate reactive prop and put the formatted date there in the setSelectedDate method.
Something like this

Answer 2

From the word "shit"?

Answer 3

Everything is very interesting. I dare to suggest that you store an object in the this.date variable, thereby you return a link! On the this.date object is visible.
Try writing something like let date = this.date ? JSON.parse(JSON.stringify(this.date)) : ...
If it works as expected, then you have an architectural error and need to think about it.
Also computed properties are cached, don't forget that.