I recall reading somewhere that the -background and -foreground options are special-cased in a few places in the Tk code and that this can cause unpredictable behavior in some cases (or something to that effect). Adding entries for the options to the call to ConfigSpecs should solve the problem:

  $self->ConfigSpecs(
      -background   => [qw/SELF Background background/],
      -foreground   => [qw/SELF Foreground foreground/],
      'DEFAULT'     => [ 'SELF' ],
  );
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