I hope you are well today and thank you for your question.
I visited your website and the header is resizing correctly for me without any issue.
Could you please test it on a different browser?
Also, This can be a child theme issue used on your site so to confirm it just temporarily use the Jevelin theme on your site. If everything works fine then it’s your site child theme issue.
This occurs in multiple browsers, but does not occur when I disable my child theme. I will do more testing on the child theme to see why it happens. Thanks.