I had the complete Pebbles set in my stash last night and today it is gone, I also had a spare belt from the same set in my on-character inventory and it also disappeared when I closed and reopened my inventory screen.
** EDIT**
All my unique items are also gone from my stash as well.
Item integrity is a mega important topic for us so if something like this happens, please use the in game bug reporting tool right away.
Is everything rolling back or just a few things?
If it's offline, it's probably steam sync getting confused.
Edit: I haven't seen reports of this so I'm not thinking it's happening a lot.
devtrackers.gg