Change to how lookups behave in Unified Interface is coming in December.
Up until now Unified Interface used Lookup View to perform the search. After the change is rolled out, Unified Interface will use Quick Find view (with resulting columns are still coming from the Lookup View), as the classic interface does.
With this upcoming change Unified Interface will be better aligned with the classic web interface. However, if you don’t do anything, the side effects could be loss of functionality (missing search columns) or performance implications (too many search columns).
For more details, see this blog article. Make follow the steps in the article to maintain your application’s lookup capabilities.