Custom SQL Lookups allow you to search by the name field in the Classic UI. This functionality does not work in the New Modern UI. We also noticed that some drop downs do not show the full list of items in the new UI but works fine in the old UI. This is happening in our Production version of 15.7 and in our Development environment of 15.9.1.
Release : 15.9.1
Component : CA PPM STUDIO
Some lookups for which customer could not search for in the Modern UX was due to that for those lookups they had value (which is essentially the id) as hidden key as as well display so we updated display value to the description which is project name and that allowed to do search as desired in Modern UX.
For the second issue where lookup values were not showing in the Modern UX, we removed ORDER BY clause by XOG and cleared clarity cache and after that we started seeing expected values in the Modern UX.