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It doesn't seem to be the exact same problem, but still this very simple example with parentheses in metavar fails to format correctly:
import argparse
parser = argparse.ArgumentParser()
parser.add_argument("inputfiles", metavar = 'input_file(s)', nargs = "+", help = 'one or more input files')
args = parser.parse_args()
-->
usage: -m [-h] input_files) [input_file(s ...]
positional arguments:
 input_file(s) one or more input files
optional arguments:
 -h, --help show this help message and exit
with the two occurences of input_file(s) being formatted wrong, but differently.