Okay, yep, I think it's the content length header. When it's working with 1.14.2, the headers look like:

< HTTP/1.1 200 OK
< Date: Wed, 21 Jan 2026 01:41:05 GMT
< Content-Type: application/json;charset=utf-8
< X-Puppet-Version: 8.24.2
< Vary: Accept-Encoding
< Transfer-Encoding: chunked

Failing with 1.15.x

< HTTP/1.1 200 OK
< Date: Wed, 21 Jan 2026 01:45:52 GMT
< Content-Type: application/json;charset=utf-8
< X-Puppet-Version: 8.24.2
< Vary: Accept-Encoding
< Content-Length: 3667

I imagine we either aren't handling the Content-Length header correctly, or the length it is computing and sending is wrong somehow.