We just completed custom domain configuration for Cloudflare Workers. The Cloudflare Worker creates a sub-request using https to an arbitrary domain endpoint outside the Cloudflare environment. This request which worked and continues to work when using our cloudflare.workers.dev endpoint, produces an error when using our custom domain. The error is too many redirects. The URL listed is the endpoint of the subrequest mentioned before.
May I ask what SSL option have you got selected under the SSL/TLS tab at Cloudflare dashboard for your domain ( Flexible, Full, Full Strict … )?
Could you switch the SSL settings it to the Full (Strict) SSL under the SSL/TLS tab of Cloudflare dashboard for your domain?