POSIX getopt(3) is very permissive of option names. Too permissive if you ask me:

A legitimate option character is any visible one
byte ascii(7) character (for which isgraph(3) would return
nonzero) that is not '-', ':', or ';'.

• https://www.man7.org/linux/man-pages/man3/getopt.3.html

ArgsParser uses getopt(3) internally to read arguments and options, so any arguments that are negative numbers are misinterpreted as options, and you get an Unrecognized option: -3 error. It's fairly rare for negative numbers to be valid arguments, but it does happen: seq is a good example.

As ArgsParser knows in advance which options are defined, it should be possible to make it treat unrecognized options which are numbers, as positional arguments instead, to then be parsed separately.