I suspect that BetterRed is correct.
Don't assume that the metadata you download always is correct. Often it isn't. Try to get a workflow where you download metadata first, and then correct it. If you start by filling things in and then download metadata you will most likely have to do a lot of double work...
Add a book, fill in ISBN using the Extract ISBN-plugin, download metadata, manually check and normalize metadata, especially series, title and author. Add extra metadata if needed.
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