That's fair but the Staging dns-persist follows draft 00 of the RFC. The challenge accounturi did not appear until draft 01 that was published just a couple weeks ago. This was mentioned in your prior thread: Dns-persist-01 deployment status and timeline - #2 by orangepizza
Let's Encrypt is very good at announcing changes. Please monitor the API announcement section of this forum. And perhaps the Client Dev section as well.
You might find this thread helpful too: Problem finding dns-persist-01 in staging