Hey Matthew,
Thanks for writing in! I have checked your site and I saw a JS error in the console:
Uncaught TypeError: $.scrollUp is not a function at HTMLDocument.<anonymous> ((index):606:6) at e (jquery.min.js?ver=3.6.1:2:30038) at t (jquery.min.js?ver=3.6.1:2:30340)
The error is coming from this JS block:
jQuery(document).ready(function($){
$.scrollUp({
scrollSpeed: 300,
animation: 'fade',
scrollText: '<span class="scroll-top"><svg width="36px" height="36px" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><defs><style>.top-icon{fill:none;stroke-linecap:round;stroke-linejoin:bevel;stroke-width:1.5px;}</style></defs><g id="ic-chevron-top"><path class="top-icon" d="M16.78,14.2l-4.11-4.11a1,1,0,0,0-1.41,0l-4,4"/></g></svg></span>',
scrollDistance: 300,
scrollTarget: ''
});
});
Try to remove this JS Code and test your Sticky Header again if it shrinks on scroll events.
Best Regards.