In my custom schema, the field user_id is of type int and the indicator field is the username. The p_any_usernames column does not get populated in Data Explorer.
When you create the custom schema definition, make sure that you have chosen the string type when declaring the field and the indicator. Otherwise, the corresponding p_any_ field will be empty in the database and using Search won't work for this field.
Only values of string type can be used as indicators.