I've had 2 equipped signets disappear (they reverted to base common quality 11 power signets) after selling other gear to the vendor. 

This happened after closing the vendor window. Re-opening it straight away also showed none of the other items available to buy back, as if I had sold nothing to her. This was all in one go, I did not close the game or do another mission before checking her inventory again. This was also after a normal campaign singleplayer mission, I was not in a multiplayer lobby.

edit: the trader also was not in the middle of a reset cycle while I was selling items, she had 20ish minutes left until reset when I checked.

edit2: exiting to main menu and back restored the signets. 

Hope this all helps in finding the root cause.