Tickets from other customers are not visible to customer users with another

[Flippo]

Hello OTRS community,

I would love a customer user to be able to see all tickets in the queues he is able to see regardless of who created them.
When a customer user logs into our customer portal they are shown all Tickets of their own customerID that are in the queues they are allowed to see.
They can not see Tickets of other customerIDs even when they arein a queue they are able to see.
The permissions are granted by group membership.

Group A owns the Queue X and is granted R/W permissions in it.
Somebody creates a ticket in queue X (CustomerID is empty in this case)
Customer user B is from customer Y and part of Group A

The customer user can not see the ticket in his overview.
when the ticket is created by another customer user from customer Y, the ticket can be seen.

CustomerGroupPermissionContext###100-CustomerID-other is active.

One workaround I could do is set the customer ID with a generic agent - this would not be pretty, but would work. There is gotta be something better though.
Do you have any indication where I could look for my mistake?
We are using OTRS 6.0.17

Thanks, Philip