Hi @Robert Sawyer,
"select the entry and drag down using the square icon" is used to move items as per my knowledge. To copy items, users need to select an item and press "ctrl+c", and press "ctrl+v".
By the way, if there are some column types like "date and time", "metadata", "Person or group" and etc. in your list, the functions will not work. This is not a bug but currently not supported by SharePoint in "Edit in grid view" function.
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