Spotify API always returns 429 on some audiobook endpoints

[SinisterBeard]

Certain endpoints on Spotify audiobooks API always return a 429, regardless of how many calls I've made. E.g. https://api.spotify.com/v1/audiobooks?ids=16rRc378Jq4lwl5t64CdA3&access_token=mytokenhere always returns a 429, as does https://api.spotify.com/v1/audiobooks/16rRc378Jq4lwl5t64CdA3/chapters?access_token=urtoken. (For the avoidance of doubt, I am including the actual token not these holder tokens). What's up with that?