It would help of you posted some screen shots as evidence of your claims that save to disk truncates your file names.
The Save to disk template defines how to construct save to disk paths and file names from the metadata.
Example - in the following screenshot you see a Russian version of Tolstoy's War and Peace in my Test library and the contents of the library book folder - note the transformation of the title and author names from Cyrillic into Latin-1.
These are my Save to disk preferences:
When I do a Save to disk this is what I get in my Saved Books folder :
Unfortunately I don't have an example of a truncated library file name - they a usually caused by subtitles, straplines, series names etc concatenated with the title in the title column.
BR