If a lookup field is "filtered-by-related" by default AND users are allowed to remove the filter, in the web interface, users are able to look up more records on a look-up field, uncheck "filter-by-related" box, and look up all records. Users are not able to do this in Unified Interface. The "filter-by-related" must be unchecked by default in order to allow users to look up all records in the Unified Interface. I opened a Microsoft Support ticket for this: 119081224004784. Engineers acknowledged this is a feature gap.