Did you check all your characters, in backpacks, the homestead chests, on companions you may have sent to the hall of heroes if you swapped any out, etc? Did you go back to where you killed Boreas and hit the Alt key, to make sure it wasn't still there?
Do you have a recent save you can reload and check for the staff, or re-kill Boreas?
If you email your latest save, and one before defeating Boreas, if available, to supportdos@larian.com, I'll take a look at it. Each save is a folder in the folder below, and can be zipped by right clicking it/them and selecting Compress:
~/Documents/Larian Studios/Divinity Original Sin/PlayerProfiles/<ProfileName>/Savegames