Some attachments disappear after a few days

Hi all, I experience always a loss of file attachments (jpg, gif, png figures, ppt, txt, pdf files for example) and hence broken links and images in xwiki pages. Errors are

java.io.FileNotFoundException:... - for disappeared figures

a blank white page - for disappeared pdf files

I have not find a right reason for that, even if i checked already ticket XWIKI-15310 , and the only way to have attachments back is to restart the xwiki service and server. This works till the next issue after some days. I am wondering if there is a solution (or a correct way to try to approach to a solution for this). I suppose in v 9.4 files are all saved in the database (not in the file system). Thank you very much for the attention.