Menu button auto-scrolls to top when pressed + Scroll to Top change button icon.

Solved:

Both problems were in primary-navigation.js

1.

if ( mobileButton ) {
            mobileButton.onclick = function() {
                wrapper.classList.toggle( id + '-navigation-open' );
                wrapper.classList.toggle( 'lock-scrolling' );
                twentytwentyoneToggleAriaExpanded( mobileButton );
                mobileButton.focus();
            };
        }

Change
wrapper.classList.toggle( 'lock-scrolling' );
to
//wrapper.classList.toggle( 'lock-scrolling' );

Lock scrolling makes the page to scroll up when you press the menu button. Deactivating it, you solve that problem.

That change may trigger some glitches because the scrolling on the page is not locked. At first sight, it’s not making any problem on the front-end. It may provoke a “double” scroll bar in PC screens if the menu is too large, not in my case. On tablets and phones, that problem won’t show up. Scrolling in the menu is working as expected.

2.

document.addEventListener( 'click', function( event ) {
            // If target onclick is <a> with # within the href attribute
            if ( event.target.hash && event.target.hash.includes( '#' ) ) {
                wrapper.classList.remove( id + '-navigation-open', 'lock-scrolling' );
                twentytwentyoneToggleAriaExpanded( mobileButton );
                // Wait 550 and scroll to the anchor.
                setTimeout(function () {
                    var anchor = document.getElementById(event.target.hash.slice(1));
                    anchor.scrollIntoView();
                }, 550);
            }
        }

Change
document.addEventListener( 'click', function( event ) {
to
document.querySelector('.primary-menu-container').addEventListener( 'click', function( event ) {

The function was being activated all the time, and that function should only work when the mobile menu is open. Now, you can use #anchors to move on the same page off the menu without “glitching” the menu icon.

That change is tested and working without any problems.