We understand that from version 15.9.3, it is possible to configure the columns which get displayed while searching for data in lists within the Modern UX. However, this functionality does not seem to be working for us.
Version: 15.9.3.
The columns to be shown were being configured in the "Browse Window" tab of dynamic lookups.
The columns to be displayed have to be configured in the "Auto Suggest" tab, under the "Attribute Suggestion List Display" section of the dynamic lookups' configuration.