Unfortunately the problem consists in case of normal sub-group too :(.

If you complete my previous example code with the next code, then the problem can be seen:

sub_group = global_group.add_argument_group()
sub_group.add_argument("--sub-arg1")
sub_group.add_argument("--sub-arg2")

Part of unexpected output:

optional arguments:
  -h, --help            show this help message and exit
  --sub-arg1 SUB_ARG1
  --sub-arg2 SUB_ARG2
  --child-arg1 CHILD_ARG1
  --child-arg2 CHILD_ARG2



Is there a patch for it too? :)