This issue is created from a comment on #2936878: Umami Theme - follow-up - margin/padding left/right for search page elements
Specifically https://www.drupal.org/project/drupal/issues/2936878#comment-12468176
===
I did see this oddity with the search button though on mobile. Ideally, the button would be disconnected from the field on mobile and set at full width. This is because mobile devices need bigger "touch targets" for a variety of accessibility and UX reasons. Attaching a proposed mock-up of how this could look.
===
Current:
Proposed: