Hello, thank you for your report.

This object is "Mesh Data" and cannot be deleted directly by selecting it in the Scene Manager and pressing Delete. (Right-clicking on Mesh Data should not display the Delete option. This issue has already been reported to the development team.)
If you want to delete it, first select it and then execute "Convert to Sub-item" in the Modify panel. This will convert the Mesh Data into a Sub-item, allowing you to delete it.
Note: If converting the current Mesh Data to a Sub-item would result in the Parent Item having no Mesh left, the operation cannot be performed. Each Parent Item must have at least one Mesh Data.

Thank you.
Tina