CloudFront doesn't seem to be hitting my backend at all

I've got a CloudFront distribution with a custom origin defined. When I hit the CloudFront distribution, it returns a 200 and an empty response.

< HTTP/2 200 
< content-type: application/json
< content-length: 4
< date: Fri, 16 Sep 2022 22:20:57 GMT
< x-amzn-requestid: d69e704c-114d-4bc9-9e09-e13a6c7bfd16
< x-amz-apigw-id: Ykt_gEMKFiAFZhw=
< cache-control: no-store
< x-amzn-trace-id: Root=1-6324f6c9-097c1ba13e2f509623b719ee;Sampled=0
< x-cache: Miss from cloudfront
< via: 1.1 b3dc72c60418e8887de31f772538f118.cloudfront.net (CloudFront)
< x-amz-cf-pop: FRA56-C1
< x-amz-cf-id: F-cU2kxROk9jWk61DMa7WSZ-V04FL43uZnDCxGloujKMQnoY68JPKQ==
< 
* TLSv1.2 (IN), TLS header, Supplemental data (23):
* Connection #0 to host my.cloudfront.com left intact

When I hit the origin directly, I get a 500 and an error.

< HTTP/2 500 
< date: Fri, 16 Sep 2022 22:22:50 GMT
< content-type: text/plain
< content-length: 654
< x-amzn-requestid: 5a1e61a6-c8e0-4014-9b22-7b621379c7c9
< x-amz-apigw-id: YkuRDEL1liAFkdQ=
< x-amzn-trace-id: Root=1-6324f739-5ee2c4f27e3f986653b634ad;Sampled=0
< 
Traceback (most recent call last):
  File "/var/task/chalice/app.py", line 1913, in _get_view_function_response
    response = view_function(**function_args)
  File "/var/task/app.py", line 16, in _exec
    r = func(*args, **kw)
  File "/var/task/app.py", line 34, in login
    token = get_token(code)
  File "/var/task/chalicelib/cognito.py", line 36, in get_token
    response.raise_for_status()
  File "/var/task/requests/models.py", line 1021, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 400 Client Error: Bad Request for url: https://mybackend.auth.us-east-1.amazoncognito.com/oauth2/token
* TLSv1.2 (IN), TLS header, Supplemental data (23):
* Connection #0 to host my.origin.com left intact

I don't know how to check if it's edge or regional; the price class is "Use only North America and Europe" if that helps. And I'm not using Cognito with CloudFront — that's integrated into the Chalice app I'm running, and I expect the error. I just don't understand why it's not propagating to the front end.

The origin domain is set correctly, I have no custom error pages defined, and the routing seems to be correct — I get different responses if I hit routes that don't exist.

What's going on?