Fix cross join when creating assignee lists

What does this MR do and why?

Splits the query into two so we don't join between the members and users table.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #417465 (closed)