The mongod/shard command path currently specifies legacy dbname.$cmd style namespaces when constructing CurOps (see here), even though most commands specify an actual namespace. There's an old comment mentioning that this is done in order to reuse existing logic for filling in CurOp that existed for OP_QUERY commands, but that appears to be well out of date by now. The router path only specifies namespaces like this when the command in question doesn't have a collection associated with it. Otherwise it records the namespace as specified by the command. (see here)

We should update the mongod/shard path to match the router path so that namespaces are properly reported by $currentOp.