User are not searchable on assignee field when assigned to a shared group (assignee search dropdown does not return any result)

Issue Summary

User are not searchable on assignee field when assigned to a shared group. The issue reproducible on Jira software versions >= 8.19.1.

This is reproducible on Data Center: Yes

Steps to Reproduce

1. Create Group with name "Shared Group"
2. Add Some users to this group e.g. User1, User2
3. Create Check Boxes Custom field "Shared with" with value "Shared Group"
4. Add Assignable User Permission to Group Custom Field "Shared With" and remove other assignable user permissions if present.
5. Create issue with selected checkbox "Shared Group"
6. Edit issue and start typing username (User) in the assignee Field

Expected Results

User1, User2 are displayed in the suggestions and can be selected as assignee.

Actual Results

User1, User2 are not displayed in the suggestions

Workaround

Set system Property -Dcom.atlassian.jira.use.optimised.user.lookup=false and restart the Jira instance.