What is the issue with the URL Standard?

When updating jsdom/tr46 to the latest revision of TR46, I implemented this new line of the label validity criteria:

If not CheckHyphens, the label must not begin with “xn--”.

This causes my test suite to fail due to these lines from IdnaTestV2.json:

  {
    "comment": "V2 (ignored)",
    "input": "xn--xn--a--gua.pt",
    "output": "xn--xn--a--gua.pt"
  },

which correspond to these lines from IdnaTestV2.txt:

xn--xn--a--gua.pt; xn--a-ä.pt; [V2]; xn--xn--a--gua.pt; ; ;  # xn--a-ä.pt

I can't tell whether this is a problem with the source data, or with our conversion script. The conversion script seems to be trying to do something with the V2 error codes, but I'm not sure exactly what.

Note that other V2 error codes in the test data don't seem to cause problems.