Recently I have seen an issue where user was unable to delete a folder in a SharePoint online document library because a file was checked out for editing by a user. An in folder there was not file with pending checkout. And mandatory checkout was disabled on the document library.
There were mandatory fields metadata fields on the SharePoint document library and documents were copied in bulk in folder view and due to mandatory fields, documents status automatically changes to checked out to the current user. As you know that a document will not be shown to the users until unless it should have atleast one minor version. So, there can be another issue that users were unable to view uploaded content.
Now how to verify that, go to the document library settings and click on Manage files which have no checked in version link. This will show you all the files without checked in version.
You can either asked the person to check in these files or can take the owner ship of the files and check in by yourself.
One important thing is that you need to provide the mandatory metadata fields values. In my scenario, I have to delete folders so I have remove the required field check from the list columns and checked in all files and after that changed the status of list columns to required (as it was originally set).
If you are copying bulk data in document library, then you have to provide the metadata for mandaoty fields. But if you want to show documents to business users so they can update the metadata fields then you need to disable the required fields and enable it again after copying all data.