On Facebook, when someone types my site: harlequinsheepsociety.com into a post shows a preview as a 404 but if you click the error message, the live site comes up. Why has this just started?

Hi, I prepared a customized 404 page in NicePage "Templates - Built-in Templates" zone. It perfectly works in when I upload the site to nicepage.io domain, but it is not working when I export it in...