Metadata edits (on the form) do not automatically calculate Author (or Title sort) Values. There is a icon to the right side to do that.
I assume Kovid reasoned there were times that value needed
help that Auto could not provide (these boxes show in Red when the Auto value does not match what is there).
If you have a lot with this issue, Quality Check Plugin
That has a check for the wrong (Auto version) sorts. It returns with the problem books shown. You select (highlight) those you want to fix, Tap 'E' (the MDE in bulk mode) and tick : Automatically set Author sort (assume that was the check you ran): OK