Currently the universalSectionTemplate for the vertical-grid page-config.json template is set to standard. We have updated the default to be grid-three-columns for the universal search layout to match the vertical search layout.
Upgrade Implications:
The new default "universalSectionTemplate": "grid-three-columns" will only be applied to net-new vertical-grid pages you add after you upgrade your theme. Feel free to customize this default or change the universalSectionTemplate property to this suggested default on existing pages.
Additionally, if you forked templates/vertical-grid/page-config.json, you will need to refresh the file to get this update for future vertical-grid pages.