so it has to be the TAP Formatter

No. Looks like it its from https://metacpan.org/source/WONKO/TAP-Harness-Archive-0.14/lib/TAP/Harness/Archive.pm

        # if it's not a format we understand, or it's not a directory
        $class->_croak("Archive is not a known format type!")
          unless $format && $ARCHIVE_TYPES{$format};
my (%ARCHIVE_TYPES, @ARCHIVE_EXTENSIONS);
BEGIN {
    %ARCHIVE_TYPES = (
        'tar'    => 'tar',
        'tar.gz' => 'tar.gz',
        'tgz'    => 'tar.gz',
    );
    @ARCHIVE_EXTENSIONS = map { ".$_" } keys %ARCHIVE_TYPES;
}
[download]

So its your xargs invocatio or its the extra -a/--archive option you're not showing

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