There is a scheduling bug which results in issues being scheduled for a wrong person and surfaces under the following circumstances:

• It concerns only sub-tasks having either an assignee in Portfolio or Jira assignees with a Jira assignee level above or equal sub-tasks.
• There must be a certain number of possible member/sub-task assignments (which resource can be assigned to which sub-task).

Workaround:
Try reducing the possible assignments of sub-tasks. That means that if each of sub-tasks has either an assignee in Jira or an assignee in Portfolio, the problem will not appear.