[stumped]

been using c-s for ages on 2 tablets but I have a couple of issues
on my newer tablet [ android 9 i think ] - all seems good except that I cannot seem to select an SD card location as default download directory - it will only use main internal storage

on my older android 7 lenovo tablet, every download gets an unwanted suffix - a long string of random characters , added to end of filename. a dash then maybe 16 characters every time, is suffixed.
so if"book title - book author.epub" is downloaded by calibre sync - from dropbox to device - ,it arrives as
"book title - book author-garbage character string.epub"
same with pdf.
its more noticable in pdf as the variosu pdf readers display that additional nonsense, whereas moon reader will hide it , for epubs.
this is before sync invokes a reader app, its at the download stage, before the "open" stage.
i wasted ages trying to figure out why. i guess its just how the app works in android 7 and is not fixable ?
changing the default download location does not change this behaviour. The tablet has an sd card which has been adopted for additional storage, so I see a single large storage area.
if i do the apps job for it - i.e. go to calibre library in dropbox using the installed dropbox android app, , select a book file, export it via save to device, then the filename is not messed with by dropbox or android, it works like it should.