Most, if not everyone, has encountered this bug even after the most recent update (3/21/2019). I have found a way to avoid having your manastones/manastone slots disappearing when you log out and log back into the game. The process is simple. 1. Open the manastone slots on the selected item. 2. Place the manastones. 3. Unequip/place the newly-slotted and socketed item back into the inventory. 4. Log out. 5. Log back in. This seemed to have worked for me. Feel free to test it out and see if it works for you as well.