I changed that parameter as noted and restarted the system. When Pi-hole is set to use unbound only, I get the curl error.
If I add another DNS, such as Cloudflare in addition to the custom one for Unbound, or remove Unbound and just use Cloudflare, the cURL issue does not present.
UPDATE: Just for fun, I tried only adding Cloudflare for ipV6 and this did not correct the issue. It still timed out on cURL in the header.