Display only Member under Assignee field based on Assignment Group

We have SM 9.51 with MSSQL DB.
We want to see only those members (say 123, 456) under "Assignee" field based on "Assignement Group"(say XYZ).

Have created New Assignment Group under which added Service Desk Agent (say 123, 456).
Now post configuration of same, when new interaction ticket is getting raised from ESS or non-ESS users, default assignment group (Say XYZ) is getting set on ticket (its as per requirement). However randomly any in-buit operators are getting set under "Assignee" field automatically instead of having an option to view only member (Say 123, 456) which are part of Assignment Group (XYZ). Also if we remove operator value from Assignee field, and try to browse members, its giving no popup to select members.

Pls help to achive this requirement.