Hi @dpbloom and welcome to the LE community forum 
Cloudflare will usually be set to redirect HTTP to HTTPS.
This may throw off the ACME client - if it expects the challenge request as HTTP.
As shown by:
curl -Ii http://home.pittfanatic.com/.well-known/acme-challenge/Test_File-1234
HTTP/1.1 301 Moved Permanently
Date: Sun, 02 Jan 2022 19:36:20 GMT
Connection: keep-alive
Cache-Control: max-age=3600
Expires: Sun, 02 Jan 2022 20:36:20 GMT
Location: https://home.pittfanatic.com/.well-known/acme-challenge/Test_File-1234