Sorry to bother you, I've just realized after quite a lot of time troubleshooting that it's a classic PEBKAC and the issue was in my request, not in Wiremock. I was sending the headers in the body, and not in headers.
Verbose logging was logging the body but I thought it was logging headers so I didn't realize I was using my HTTP client wrong. I'm just gonna delete this post at this point 😂 😭