According to my New 3DS browser, it can't confirm the site's security certificate. While this is consistent every time I visit the site on this device now, it hasn't happened before about this time last week, even here.

While the prompt is only a very minor inconvenience, I can't help but wonder if something's expired, perhaps leaving a potential security hole, so I thought I should let you guys know.

Unfortunately it's a known issue with LetsEncrypt security certificates which MG uses:

https://letsencrypt.org/docs/certificate-compatibility/

I won't be changing the certificates again any time soon so it's up to Nintendo to put out a firmware update for it to go away.