Based on my testing of this option in Portfolio for Jira with customers, I think that the sub-task Option Issue Import Level should be removed and documentation improved.

In summary here is the issue and the recommendation:

Portfolio for Jira has an option called Issue Import Level for building the schedule. There are a number of possible options and this feedback is related specifically to the Sub-Task option.

In summary:
When you set the option for Issue Assignee Import Level to Story, Portfolio for Jira correctly handles this Use case:
1) Story with no sub-tasks - it imports the actual Jira assignee into Portfolio
2) Story with Sub-tasks - it imports the assignee on the Sub-tasks

When you set the option for Issue Assignee Import Level to Sub-Tasks, Portfolio for Jira seems to get confused:
1) Story with no sub-tasks - it FAILS to import the actual Jira assignee into Portfolio
2) Story with Sub-tasks - it imports the assignee on the Sub-tasks

Based on this logic, I think that the Sub-Task option should be removed since it provides no value and handles a very limited USE case that is not representative of what happens in the real world. The Sub-Task option is assuming that if a Team uses Sub-Task it will have Sub-Tasks for EVERY story. This is a very Corner Case which I think is Extremely rare based on the realities of what teams do on a daily basis. I have yet to find a team that uses Sub-Tasks so diligently that they will create Sub-Tasks for every Story.

Therefore, why have an option that brings no real value to the product? I propose that this Option be removed. It only causes confusion to your end-users. If not you should at least document how this option works and the assumptions that the Portfolio makes around this option.

One final comment on the Documentation you have at this link:

https://confluence.atlassian.com/jiraportfolioserver/mapping-issue-assignees-to-team-members-868977966.html

It is great to talk about EPIC's and Stories but you don't cover the next level down which is where the problem is. The customers I work with set this value to Story and Portfolio is correctly handling even the next level down into Sub-Tasks. There is nothing in your documentation about what to expect when a customer uses a mix of issue Hierarchies in the SAME Jira project:

1) EPIC-Stories,

2) EPIC-Stories-Sub-Tasks,

3) Stories-Sub-Tasks

which is what happens in the industry all the time. This is actually working correctly now when you use the Story and EPIC Issue Assignee Import level settings and I recommend documenting it.