Epic estimates overwritten on import by un-estimated stories

We are now using Portfolio to plan a cross-team roadmap and have discovered an issue with importing stories into an Epic.

We use Epics to describe each Feature (and grouped into higher Initiatives) and the Epic can be worked on by one or more teams. Our process working with Epics and Stories via Portfolio goes as follows.

1. Epics are defined in JIRA
2. Epics are imported into Portfolio
3. Epic estimates are entered into Portfolio representing the best guess per team to allow long-term planning.
4. As each Epic is further defined and gets closer to the top of the plan, the teams created their stories (in JIRA) under the Epic
5. The teams then estimate and allocate story points to the stories (in JIRA)
6. Once all teams have created and pointed all stories for the Epic, they are imported into Portfolio, replacing all Epic estimates with the total of the story points.

We have discovered a problem between steps 4,5 and 6 above. If we ask the teams to create stories and before step 6 is complete and Update plan is run, Portfolio will try to replace the Epic estimate with 0, which I presume is the total of all stories.

For example:
*Epic impacts App and Server teams
*Epic estimates are App(20), Server (30)
*Server team add 3 stories under the Epic (no points)

• Update plan is run and all Epic estimates are replaced with 0

Is this expected behaviour (and therefore a problem with our process) or a bug?