For a reference field on a form, searching for a value and clicking on reference lookup, the search takes 'Starts With' instead of 'Contains'.
(Eg : For assignment group field on incident record, on entering fo, two valid results are shown. Clicking on reference lookup shows Name starts with 'fo' and not Name contains 'fo').
Any supported release
The system property 'glide.ui.goto_use_contains' controls the search in the list view and not reference lookup.
A system level user preference with the format <table_name>.autocomplete.contains with value true has to be created for contains search to work on reference fields.
For assignment group field, User Preference with below details has to be created:
Type : true|false
Value : true
System : true
Once the user preference is created, make sure to clear the instance cache (by doing cache.do) and login back to the instance.