Dear Alejandro,
Good day! Thanks for posting in the community. We are happy to help you.
According to your description, for the search bar, I assume you mean the search box placed at the top of SharePoint. If so, I would like to collect more information to troubleshoot the situation.
- Since the situation only happens to specific users, I would like to know how they searched (search on the SharePoint Home page or go to the site/library/list, then search) and what keyword they used. Please note that the modern search experience in SharePoint is a personalized experience. If these users don't have permission to files/folders, they can't find them in the search results.
- Please share the screenshots of the search results on your side and on these users' sides. Please click the Insert Image button to insert them.
We look forward to your response. Thanks for your cooperation.
Sincerely,
George | Microsoft Community Moderator