There is an issue when shift clicking the travelers sack from your equipment into your inventory, if you have an open slot one space away from the edge, the traveler sack will fill that space while also shrinking your inventory, thus making your traveler sack inaccessible, until you wear another one, then it shows back up in the same spot that was hidden before.
*Of course, you can scroll wheel into another container, and it will eventually pop into that new container.