A RESTCONF script fails on one router with 409, then 400

A Python script creates Loopback100 with a /32 address on 20 IOS-XE routers through RESTCONF. It worked on 19. On R7 the POST returned 409 Conflict. A colleague changed the script to use PUT "to overwrite it", and now R7 returns 400 Bad Request. The other routers are fine. You are asked to make the script work everywhere and be safe to re-run.