If an application is launched in the Modern theme, the search button does not appear in the toolbar.
This is due to changes made to the tl:TabSwitchCommandModelConfiguration via Ticket #26373. Previously, an image for the button could be configured in the tl:TabSwitchCommandModelConfiguration. Previously, IMAGE_BUTTONS_SEARCH_SIMPLE was configured as the icon for the search. Following the changes made by Ticket #26373, the icon from the arguments in legacyTabbar.template.xml is now used.
The search layout is defined in searchInput.layout.xml. The icon for the button is IMAGE_BUTTONS_SEARCH22. In the Modern Theme, the variable IMAGE_BUTTONS_SEARCH22 does not exist, so tl:ThemeImage$NoIcon is used, which adds style="display:none" when the button is rendered. As a result, no search button is displayed.
Test
Launch the demo in the Modern theme and check whether the search button is visible.