As far as I know Calibre does NOT change the value that is set in the Edit Metadata screen - at least it never does for me! However if you download metadata and allow Calibre to overwrite the author field it will re-calculate the author sort field. This will show up before you press the OK button and not 'behind-the-scenes'
A better way to handle this is to use the Tags Browser to find the Author and edit the Author Sort value there. That will then become the new default value if you add any new books by that author.
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