MOBI/AZW3 format only supports the "author_sort" metadata, and not an independent "author" filed. So the author_sort gets filled in, and where usually the "author" field fills in the Author in calibre, the mobi/azw3 imports nothing there.
calibre auto-generates the "author" field directly from the author-sort, instead of flipping it around, which I have suggested once to Kovid. But he vetoed the idea, with some complicated rationale that I didn't really understand.
I use the
Quality Check plugin so I can flip them around with a shortcut key.