Thats because you have the Mobile Header enabled in Customizer > Layout > Header.
This removes the Header Widget. You would need to disable this for it to work.
You would also need to adjust CSS for mobile and desktop:
/* Mobile */
@media(min-width: 768px) {
.header-widget .custom-html-widget .adsbygoogle {
width: auto;
max-width: 640px;
}
}
/* Desktop */
@media(min-width: 769px) {
.header-widget .custom-html-widget .adsbygoogle {
min-width: 640px;
}
}
Adjust the Width properties accordingly.