Hey @robm did you ever figure this one out.
This is a repeating issue in Mautic and I think once we figure out what the main issues are we need to document it, I will even write it up for the Mautic Documentation. I have seen so many posts on this and written so many of my own.
Also we have written another post on this (@rcheesley - maybe worth merging them [Anonymous Contact to Known User After Form Submit. Chrome/Safari don't work, Firefox works?]) and again here we are seeing that Chrome and Safari are not converting users to known users, however Firefox does
Can someone try and help with troubleshooting this and what the steps are to begin troubleshooting 
- I have checked both /var/logs/*
- I have checked web server logs /nginx/mautic.error mautic.access logs
- I have looked in the console logs
- Verified CORS is enabled
Any other thoughts ?