According to my CRM instance (on-prem 2015) the Contact field is filtered by the related Company Name from the Customer field and the option to uncheck this option is disabled in the Lookup more Records.
I found https://community.dynamics.com/crm/f/117/t/164553 which offered a solution, but when I edit the Contact field's properties in the Form Editor, thw 'Related Records Filtering' already has the 'Only show records where:' box unchecked.
I also read https://social.microsoft.com/Forums/en-US/13ff48ee-e23e-49f2-a683-2c4669bc828e/unable-to-uncheck-the-box-for-filter-by-related-company-name?forum=crm where other people are also having similar trouble.
I'm hoping the difference is that the first thread is referencing CRM Online from last June and that maybe it is fixed in On-Prem 2016?