Okay, I tried a fetch with WP_DEBUG enabled. Initially there was a warning from the Block Biggbar plugin, but when I disable that and the warning goes away the endpoint still returns an empty body.
Caching! You nailed it! I was using the “Jetpack Boost” plugin; when I disabled that the JSON body came back.
Does your Site Health page give any errors? I have a similar problem on my site; WebFinger is reporting properly, but the main author page doesn’t return json, it just returns a link to the endpoint with json.
