After spending lot of time, I looked into the code & found that, the regex pattern is always matching for only registries pointing to npm. In my case we have a custom artifactory the URL that looks like //art01.spotifycloud.io

Suggested solution

we can generalized the Regex and avoid hardcoding

const authLine = userNpmrcContent.split("\n").find((line) => {
  // Dynamically adapt to any registry by looking for :_authToken= pattern
  return /^\s*\/\/.*\/:[_-]authToken=/i.test(line);
});