Restart Calibre and you get 'natural order' (the date added in this case) ascending (latest at bottom)
There is a sort tweak, but it affects ALL sorts.
Code:
ID: maximum_resort_levels
Set the maximum number of sort 'levels' that calibre will use to resort the library after certain operations such as searches or device insertion. Each sort level adds a performance penalty. If the database is large (thousands of books) the penalty might be noticeable. If you are not concerned about multi- level sorts, and if you are seeing a slowdown, reduce the value of this tweak.