Sometimes a two-part field authors appears even though I have it set to one-part. There are no dots showing the menu to change. You have to enter something, dots will appear and you can change it to a single partial field. I don't see any correlation when the problem occurs, but I have it every dozen or so positions.

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https://s3.amazonaws.com/zotero.org/images/forums/u8926445/65ir1yeestx12ya0thde.jpg