Hi Hochen,
Seems you are trying to use SonarQube server for existing code. I did the same here I am sharing what will happen to issue assignment. These are the results of my experience
I created system with LDAP config using SAML. I ran analysis when no user was logged in. Result is issues were created without assignment. The reason is SonarQube maintains its own users table in DB. While analysis if user found in that DB it wil assign it to that user.
Issues will be assigned if there is no conflict like issues could be on line or on file. If its on the line then its easy for SonarQube to identify user and assign the issue. But in case there is conflict like file is not covered for the configured threshhold and multiple people changed the file then, issue is created on file without assignment.
So anyway if user found in future and its clear who introduced it will get assigned to him. Just there is one thing that, you will have to ask your developers to login once to SonarQube server so their account entry will be there in Users table in SonarQube DB and issues will be assinged to them if inroduced.