Ignoring unknown options in the configuration file is analogous to
ignoring unknown tags in html: it makes it easier to use the same config
file with multiple versions of the logger, with older versions ignoring
the options they don't understand.  This is a tradeoff, and I suppose it
could be argued that the tradeoff is being made in the wrong direction
here.  I don't use configuration files for logging myself, so I'm not a
good person to render an opinion on that.

But IMO this is not a bug; changing the behavior would be a feature request.