I just finished putting Pihole on a fresh rpi. After getting it running, I added cloudflared using the instructions here. Testing it with wget seemed to work fine, but trying https://1.1.1.1/help showed DOH was not active. I tried several fixes mentioned here and on Reddit, but got the same result. Then I realized I could better confirm if was working correctly by looking at the connections on my router. Sure enough, everything coming from the Pihole rpi was to port 443. I could also modify DNS settings on the Pihole, and immediately see traffic going to port 53. So I'm not sure using https://1.1.1.1/help is the best test for all configurations.
That test is something offered by Cloudflare and is not referenced in the Pi-Hole guide for Cloudflared.
Sorry, yes, it's not listed in the PiHole guide. It seems to be mentioned in many other places as a way to validate.
This test is as frequently mis-applied as the DNS leak test.