Not a bug, you haven't taken enough care in ensuring the author names have consistent and correct values. This can happen if you download metadata from multiple sources, e.g. Amazon, Google and Goodreads may use slightly different values for the same author.
I would go into Manage Authors and harmonise the different "Modesitt' Author values into a single value. Where necessary you can also override the computed Author Sort value - e.g change John, St Madeleine to St John, Madeleine.
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