I think it's the same issue. A dash argument that is not clearly a number is interpreted as an optional's flag.  With few exceptions, the parser does not examine the contents of the string

It tests the initial character, it tests for space, it tests for numbers (I'm not sure scientific notation is accepted).  That's about it.

Argparse has a minimum of constrains on what is a valid flag string.  For example, the following is ok:

parser.add_argument('-1,2')
# appearing the namespace as: Namespace(**{'1,2': None})