A family of Microsoft on-premises document management and storage systems.
That is the default behavior for Document Set Version History in SharePoint Server 2016.
The version of a Document Set would start from 1.1 with draft versions enabled, and start from 2.0 with major versions enabled only.
This setting is not in written in any official document, but it has been changed and kept ever since SharePoint 2016.
In fact, document set version histories do not get recorded and managed as document/item version histories. You will need to manually create a version history for the Document Set (for it to be restored or deleted) using the “Capture Version” option.
For detailed information on how to manage version history for document set, you can refer to the document here: Create and manage Document Sets (Section: Manage versions).
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