Apparently, the styles for the flatpickr were not loaded.

Depending on how you installed `flatpickr`, you would need to include the flatpickr CSS stylesheet to resolve the issue.
You might simply need to include CSS with the <link> HTML tag:

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flatpickr/dist/flatpickr.min.css">

or add to assets:

@import 'flatpickr/dist/flatpickr.css';

I was looking to install the flatpickr with Laravel. Here are the sources I used to resolve the issue:
How do I load the CSS for flatpickr when I use npm to install it in Laravel?
Laravel + Flatpicker: How to Install using NPM