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1) In the header.php line 126 is
<a class="assistive-text" href="#content" title="<?php esc_attr_e( 'Skip to content', 'themonic' ); ?>"><?php _e( 'Skip to content', 'themonic' ); ?></a>replace with
<a class="assistive-text" href="#main" title="<?php esc_attr_e( 'Skip to content', 'themonic' ); ?>"><?php _e( 'Skip to content', 'themonic' ); ?></a>You are only replacing #content with #main
2) Archives dropdown widget is provided by the WordPress itself and not related to theme and I also quickly checked with the default Twenty Seventeen theme to rule out the theme issue, it is an issue with bundled WordPress widget. I will file bug report to WordPress and let you know once it is solved.