Since the changes described below were implemented lookup will display a list of the most recently viewed records when you click on the lookup.

Preview for usability enhancements to lookups in Unified Interface
https://powerapps.microsoft.com/en-us/blog/preview-for-usability-enhancements-to-lookups-in-unified-interface/

The most recently viewed records list is populated when a user opens a record. We would like a record to also be added to the list when it is set in a lookup even if the record itself has not been opened. From a perspective of a user a record that was set in a lookup is equally as relevant as a most recently used record as one that was actually opened.