flatten not working on csv input data

[TobyBrull]

I would have expected the following commands to give the same output.

mlr --c2j --from <( echo $'obj.a,obj.b\n1,2' ) flatten
mlr --c2j --from <( echo $'obj.a,obj.b\n1,2' ) cat then flatten
mlr --c2j --from <( echo $'obj.a,obj.b\n1,2' ) cat | mlr -j flatten

The output of the first two commands is:

[
{
  "obj": {
    "a": 1,
    "b": 2
  }
}
]

The output of the last command is:

[
{
  "obj.a": 1,
  "obj.b": 2
}
]

I would have expected the latter in all cases. Am I missing something?