Customer user rights to see ticket

[geri]

What is the exact requirements for a customer to be able to seee a certain ticket?

In a case i faced with is as follows

The ticket is in a queue assigned to group GR_A.
CustomerGroupSupport is enabled
CustomerGroupAlwaysGroups contains GR_A

Isn't this should be enough for every customer to be able to see the ticket?
The log says [Notice][Kernel::System::Ticket::TicketCustomerPermission] Permission denied (UserID: USER2 'ro' on TicketID: <id>)!

[crythias]

Customers can only see their own tickets in My Tickets (login matches what is attached to the ticket) or what tickets match CustomerID or customer_ids in Company Tickets AND group membership if CustomerGroupSupport is enabled or all groups if not enabled.

For privacy reasons, no ticket is globally visible on the customer side though you can have a public FAQ.