If shared the download link is accessible via a public url and does not require signing in with an Adobe ID, so session expiration is not the issue here.
What happened for the customers? Were they unable to access the url provided, did the download fail with an error message, did the download fail silently?
What size of file(s) are you sharing?
Would it be possible to share one or more of the links with me so we can investigate. My email is kenrice@adobe.com.