When I listen to flatpicker's on-close event and log v-model's value in the event function I get previously selected date
<flat-pickr v-model="myObject.happened_at" @on-close="onDateChange"></flat-pickr>
Finally I found a solution for this problem. You have to listen on @input event
<flat-pickr v-model="myObject.happened_at" @input="onDateChange"></flat-pickr>
As I understand @on-change event is triggered before the v-model is updated so you get previous date instead of selected one.