Cannot update custom fields

I am unable to update custom fields from my app. I've even tried using the REPL app. The original field has the value “Test Hall 5”.  As you can see below after setting the field, the response indicates it is successful.  I then click submit on the ticket to save the changes. As you can see, when calling get() the the previous value is still there. Updating other ticket fields via set() works. What am I doing wrong?

1: async request - zafClient.set('ticket.customField:custom_field_27609398553885', "Test Updated");> {}1: async response (undefinedms)> {
  "errors": {},
  "ticket.customField:custom_field_27609398553885": "Test Updated"
}2: async request - zafClient.get('ticket.customField:custom_field_27609398553885');> {}2: async response (undefinedms)> {
  "errors": {},
  "ticket.customField:custom_field_27609398553885": "Test Hall 5"
}