@
Motomaggot - some of the Preferences for Add Books settings relate to Copy to Library - a couple of them specifically so.
Have a look and see if anything there 'makes sense' apropos this problem, otherwise post a screen shot of it here.
That said - I'm not sure if any of them would cause this sort of problem
FWIW : I do what you're doing quite often, never had this problem - but... SFW
BR