Filtering on Name on a Custom Record

I have come across an issue on a Custom Record search that I'd be glad to know if anyone can throw any light on.

It's a Parent/Child Custom Record that we have created and the search is on the Child. I am trying to filter by Name (specifically Customer Name ) but the Name filter does not give a list of Customers - instead we get this:

I have verified that I am getting the right Name field (confusingly there are two fields called Name - one is the Record Name and the other the Customer Name). I have tried using a formula and I have also tried using the link to the Parent Custom Record and finding the Name from there. The Name field on the Child sources from the Parent. None of these options allow me to select from a list of Customers.