When you search for an item in SharePoint Online, no results are returned.
In this scenario, you have a SharePoint group that contains many users, and the group is used to define permissions for the resource that is missing from search results.
To resolve this issue, keep SharePoint group membership size under 2,000 users.
If you need to grant access to many individual users, use an O365 security group as an alternative. To do this, create a security group, and then add the security group to the SharePoint group instead of adding the individual users to the SharePoint group. Next, add the individual users to the security group.
For more information about security groups in Office 365, see the following Microsoft website: