Downloads with multiple files and completes, but some vanish later? - Fixed?

[there]

I have just downloaded 13GB of txt type files, but on inspection some were corrupt, so forced a recheck and they became fixed.

on next using qbittorrent found the 100% complete post now 85% complete, so forced recheck, and it completed again. This time I was more clever (but not clever enough) and I made Multipar files (but only 5% - not enough to recover)

I have checked the av log files and they have not been touched. It is not long file names. Hard disk and media check is ok.

I cant work out what is corrupting them?

Thanks

[there]

found the issue is that the ebook reader (calibre) is corrupting them

Thank you