Flags load only after refresh on checkout page

Good Day!

I am having some issues with the checkout page.
The problem begins when you choose the country that you want to order from and the flag does not change the first time.

Only when I have refreshed the page the flags did appear on the Phone form field.
At first, I thought that it was my Caching plugin but then I have tried to deactivate it and saw that the same pattern occurs.

I would be really happy if someone could help me on the matter.
Thank you very much in advance!

The page I need help with: [log in to see the link]