Ah, I see the problem. This is a bad interaction between caching and HTTP content negotiation. The WordPress ActivityPub plugin uses the post URL as its AP Article object id, which means it switches between serving JSON AS2 to ActivityPub requests and HTML to browsers based on the request's Accept header, but it's not including Accept in the Vary header to indicate that. @pfefferle@mastodon.social adding Accept to Vary should fix this!

(…on a related note, content negotiation considered harmful 😠)