Some new achievements are not returned from the API. For example, I have this achievement: https://api.guildwars2.com/v2/achievements?ids=3843
I noticed this issue here: https://metagamerscore.com/forum_topics/5086
That achievement doesn’t have any data (name/description/etc), nor does it award any AP. It’s probably just a content artifact. There isn’t currently a way to blacklist achievements on the API, but if I had time I’d blacklist it so it’s not being returned from that endpoint.