Yeah, noticed this as well. It doesn't go back to inventory :( was really sad when it happened. For me it was a bit different though, bc i had it on holgar and then replaced holgar with scarecrow and the item disappeared. Edit: Tested again and Viking horn didnt disappear this time. I dont know why it works now but not previously.
**5 months later** Hey, yeah, still happening, but I took notes: Put on Viking hat (it was the furthest right in backpack) Give Horn Take hat Both poof!