As per the Object key naming guidelines "/" is not supported for either Filename or Folder/Prefix name. Only exception is objects with a prefix of "./" must uploaded or downloaded with the AWS Command Line Interface (AWS CLI), AWS SDKs, or REST API. You cannot use the Amazon S3 console. If you are already seeing this behavior in your bucket then I would encourage you to open a ticket with AWS support so this can be checked further.
It seems that your S3 bucket has versioning enabled or suspended. The reason you are seeing the object bytes from the S3 inventory report may be due to the versioned objects, which means that the object is deleted from bucket and a versioned object is present under prefix folder1/folder2/folder3. To check this from S3 console you can enable the show version toggle button on your S3 bucket after navigating to folder1/folder2/folder3 prefix as follows. If you would like to know the resource specific details then you can open a support ticket with AWS .
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