Damn, I was hoping you would sort it out
I can't see anything wrong with what you are doing. I haven't been using the plugboard lately, so I just configured it and ran a test on my Glo. I sent 22 books for about 5 authors and five series and put them on one shelf. When I opened the shelf and chose different sorts, the sorting worked correctly.
One thing I can think of is when you open the shelf, even if the sort is shown as by author, select it again. Or select a different sort and then change back to author. I have seen some cases where the sorting didn't match what was shown, so this could be it. This seems to always happen the first time you open a new shelf. The sort shown is title, but it seems to be actually sorted by the whatever was last used for a shelf. After selecting a sort for that shelf, it is remember.
If that doesn't fix it:
- Can you take a photo?
- How does it sort in the main book list?
- Can I get a copy of the database from the device? That's a last resort, but I might be able to see what is happening.