ActivityPub vs Jetpack Boost
Sure enough, checking the URL with cURL, or an HTTP client like Thunder gave a 200 response but no response body. It had been several days since I had last made any changes behind the scenes, and everything had been working in the interim, so I was a bit stumped.
Anyway, after a bit of digging around, I came upon this thread and specifically this post which pointed me to the problem: Jetpack Boost. It wasn’t the most recent plugin I’d installed, and - as I mentioned - things had been working fine since I had installed it… but something had happened in the background which had broken things, and turning off Jetpack Boost as a first test instantly solved the problem.