As per #216, the list of subjects provided by --participant-label is supposed to be merged with the filters provided in the config file. Right now, if config filters are provided, --participant-label is just ignored.
It's possible this behaviour was intended after further discussion, I can't remember. Specifying subject filters is a bit unusual, I could see how if one is doing that, one wouldn't want --participant-label to interfere. @akhanf, do you have any memory of this?