Ah, it turns out I was just hitting the URL length limit again, and Cloudfront doesn't give a useful error message for it. My only clue was the 413 status.
Some other threads about the issue I'm now hitting:
- GraphQL Query Size Limit Reached
- Graphql in POST instead of GET
- Prismic preview not working (414 Request-URI Too Large – graphql queries too long)
- 413 Request Entity Too Large - #7 by esmith
Please fix it. There's only so far I can optimize these queries.