I think this is the default behavior of readline module.

the default word delimiters for completion contains dash(-). so completion breaks at dash.

  >>> import readline
  >>> readline.get_completer_delims()
  ' \t\n`~!@#$%^&*()-=+[{]}\\|;:\'",<>/?'

In contrast, the default word delimitors of GNU readline is "\t\n\"\\'`@$><=;|&{(" and perl binding does not change it.

when I remove dash from delims like below, it works.

   >>> readline.set_completer_delims(readline.get_completer_delims().replace('-', ''))
   >>> input()
   a-
   a-bc  a-de
   a-