I can reproduce the issue in my SharePoint Online.
When you add a vertical section in the page, no web parts are added meanwhile, it shows blank by default.
Expanding the browser to more than 100% will not display the blank vertical section. In addition, only delete the vertical section will not show blank.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.