Confused about profile url wrong for a selected few users

Hi guys.
I'm asking for advice on a specific issue I am seeing, and after days of troubleshooting have hit a brick wall.

I use sh404SEF for URLs. This will create a URL to /social/USERPROFILENAME. However, for a select few users I am seeing the URL /easysocial/USERPROFILENAME. When clicking on this, I get a 404 error.

If I type the full URL of an affected user manually (/social/USERPROFILENAME) I still get a 404 error. However, if I type it again removing the user ID that seems to get generated in the SEF, I can see the profile... i.e.

https://guys4guys.social/easysocial/378-robert82 - Fail
https://guys4guys.social/social/378-robert82 - Fail
https://guys4guys.social/social/robert82 - Pass

I've searched the sh404 URL manager, and the /easysocial path does not exist. I have no aliases, and I've even looked at the sh404sef database in phpMyAdmin. It simply doesn't exist.

I have cleared the cache down, and even went as far as completely disabling all caching, turning off CloudFlare so the website was being accessed directly with no caching. Still this odd "easysocial" url persisted.

I've been through my menus to make sure there is no menu item with the "easysocial" set as an alias (and even if it was, I would have assumed all URLs for users would get this!!).

So does anyone have any ideas on where Joomla/sh404Sef is generating this url from? It's driving me crackers trying to find it and rectify it.

Cheers for any help.
Dave