Did @jsha @schoen @josh or another LE ops see that message : 1204656 - Add ISRG / Let's Encrypt root certificate ?
Kathleen Wilson (Mozilla) said about https://helloworld.letsencrypt.org/ :
I need a test website whose SSL cert chains up to the root cert to be included.
(The current configuration doesn't match that: Helloworld.letsencrypt.org can only find certificate with DST X3 loaded - #2 by pfg)