Per the Documentation:

• Mapping issue assignees to team members

The noted Items required for the Members field to import the assignee are:

For the Jira issue assignee to show in the team member column in Portfolio for Jira, these conditions must apply:

• There is no member set in Portfolio for Jira.
• The assignee is set on the issue in Jira.
• The assignee is part of a team in your plan.
• If you've made a manual team assignment, make sure the assignee is part of that team.
• To see the assignee that is being pulled from Jira, you have to calculate your plan.
• The issue assignee import level must be set to a hierarchy level. See Configuring initiatives and other hierarchy levels.

But the requirements for calculation to associate this user is not clearly defined:

To see the assignee that is being pulled from Jira, you have to calculate your plan.

This pulls in elements from some additional documentation Scheduling and timeline, Scheduling behavior & Scheduling unestimated items to piece this behavior togather BUT does not clearly define that this also requires an estimate or a manual Sprint association for the assignee to populate in the member column with the following variables to make this true:

If Unestimated item scheduling is set to:

• Base on default estimates & a value is set for the default values (Sprint, Epic, Initiative...)
  • Assignee will be populated per the user assigned to the issue in jira as a default estimate is added to un-estimated issues to allow the calculation to occur
• Base on target dates
  • Requires Either an Estimate or Manual Sprint association
• Custom
  • Requires Either an Estimate or Manual Sprint association
• Off
  • Requires an Estimate