Not sure if it can be related or I’m understanding correctly the point but you can try to have a look at this repo:
The thing is that people registered at the above app are included as part of a SOLID Pod. If you are deploying your own Community Solid Server and you want to track users who can register themselves with different issuers maybe it’s related with this topic:
Maybe there’s a way to trace all the registration requests in your CSS (have you checked the logs when people are arriving?) I haven’t go that far yet, sorry