I get a "too many redirects" error when I send a fetch request to "https://couchdb.1paragraph.app", but only with the worker running on a custom domain, not with the worker running on the default workers.dev subdomain (or if I send the request from a browser, or from the deno repl, or from wrangler dev).

You're saying you only get it from a worker on a custom domain fetching it? Or am I missing something? That subdomain isn't proxied at all, so CF isn't really involved

I'm talking about sending a fetch request from within a worker.

Interesting.. are you doing anything else then a simple fetch? Could you link the custom domain of the worker?