FWIW - on Windows I can save this book:
With this template
{authors} - {title},
And get this:
FTR the library filename is:
_Hello_. World! - Anat Admati.epub
I must admit to being confused: I've always thought file naming rules were less restrictive on MacOS than on Windows because of its Unix heritage. Windows inherited its conventions from DOS/CPM/RT-11.
But I also thought Apple's file system was known as HFS+ - is APFS just a new badge for HFS+, or something entirely different, like MS's ReFS.
BR