Ah, here was my original question about this:
I couldn't find it because I apparently posted that from another account for some reason.
From that thread, here is my example: https://cl.ly/1U3C3k093s44
I was trying to schedule that OMNIA-6 task to start next. It absolutely refused to do that. After spending way too much time hassling with it, I gave up and planned out the sprints manually.
I have the exact same problem. I am completely unable to control the schedule that Portfolio generates, and therefore JIRA Portfolio is essentially useless. (For the Atlassians, that's issue JST-213949 in your system.)
I asked about it here and my question has apparently been deleted. Then I asked support and they advised me to sort my tasks by priority. I repeated that I had done that from the start and it had no effect on the schedule. And that was the end of it. JIRA Portfolio remains useless to us. We eliminated all of the factors listed on the Portfolio support page that might affect the scheduling:
Maybe one of those things is affecting your scheduling?
In our case, we tried sorting the tasks, epics AND initiatives to try to influence the schedule and it had no effect. Rearranging the ranking will cause the schedule to recalculate, and things will move around. But we have been unable to make it schedule the next priority as the next task on the schedule. So we still schedule sprints manually and we'll probably discontinue JIRA Portfolio since it doesn't work.