What's the logic behind the mortar becoming unusable by just being moved?
The mortar can not be used when disassembled. The mortar is transported in disassembled state (typically in 3 packs), as a consequence it can not be used when transported.
It is also obvious if at least 1 pack is not delivered to the assembly site, for example if the one who carried it was killed on the way. The mortar will not be able to assemble and it will be unusable.