What tickets can the customer see that are not his tickets? Is the customer able to see ALL tickets in the system? Certain random tickets that don't belong to him? Inability to see a different list between "My Tickets" and "Company Tickets"?
This is hard to troubleshoot as stated in your request.
What doesn't work?
What is expected?
What is experienced?
What has changed?
What problems are caused by this?
How long has it been happening?
Do other customers have this issue?
Are there any logs that indicate issues?
Can you replicate the issue internally?
How would it be possible for random viewer of this list to replicate your experience?