On Linux the default cache directory is /tmp/hugo_cache. This is a volatile location, cleared on reboot.
Set the HUGO_CACHEDIR environment variable to something other than the system temporary directory, but make sure that it is outside of your project path. Related comments:
- https://discourse.gohugo.io/t/hugo-modules-for-dummies/20758/8
- https://discourse.gohugo.io/t/module-cache/34091/2
Without knowing all the details, I don’t know if this will resolve the problems your users are experiencing, but it’s a wise move nevertheless.