Map_fail_uri: "/register" stopped working

I have authentication setup with mellon and map_fail_uri: “/register” set, this used to work but now users who do not yet have local accounts get the message

Error – can’t find user for username
Run ‘nginx_stage --help’ to see a full list of available command line options.

Instead of being redirected to /register.

I can not see why this is happening. I’ve set lua_log_level: ‘debug’ and I see
[lua:debug] [pid 32721] @/opt/ood/mod_ood_proxy/lib/ood/user_map.lua(21): [client] Mapped ‘username’ => ‘username’ [0.011 ms]

Not sure where to look next

please delete this as its a duplicate of Redirect not happening when user does not exist

I don’t seem to be able to delete my own topic