Let me just create a new topic for this issue so that Blizzard can take notice. Yes, post the latest patch, this problem still persists: player 2’s inventory resets each time player 1 (the host player in couch co-op) interacts with the town’s merchants and service people (Occultist, Blacksmith, Curiosity Vendor, etc.).
I hope this gets fixed in season 15 (or sooner), as it really causes irreversible accidents, especially when player 2 is using the Blacksmith’s salvage service.