It is mostly about TIMING, so the only way I could see the current structure of Calibre 'playing nice', is to slow it way down.
<the screams heard round the world: NOOOOOOOOOOOO >
Things have to happen in order. NAS/share operations are typically buffered at either end to make performance faster (but not necessarily as requested).
If you did ever peek (Never touch), you see 1 or more folders must be created when you add
a book to calibre
- Authors / Title (id) + OPF file + cover file / format(s) file
- and the metadata DB entries
(I do not know the exact order, but obviously the folders need to be there to put something into them
. Buffering can delay one of those steps )