Can confirm this bug is real. I've lost items from my quick slots and items from my inventory when I switch companions. I've already lost the unique pistol Forgiveness and Iridescent Scarab Figurine, was bummed about that. There is no way to get them back without using the cheats, and I don't want to lose achievements by using cheats to bring back items.
Does anyone know if removing all items from a companion before removing them from the party will prevent this issue?