Regading the MongoDB documentation about jumbo chunks, MongoDB removes the jumbo flag upon successful split of the chunk. That's not the actual behaviour, though.

The current behaviour is that after a successful split of a jumbo chunk we get 2 new jumbo chunks because new chunks are initialised as a copy of the original chunk.

Since there is no test controlling that, the scope of this ticket should also include a test case to cover that scenario.
You will find attached to this ticket a brand new test file to force the issue. It must be run on the sharding suite.