Entity Form Metadata - no error - fail

Hi, I'm unable to make Web Form Metadata work in a 7.0.0.23 portal on a crm 2016.

The set up is very straightforward and the web form creates the underlying entity no problem, but both the update on save and prepopulate fail to work. New sessions are being created too. I've tried with and without authentication....

Config is to pick up the contact / portal users account and set it on a custom entity.

Any gotchas that i might have missed?

Phil