Moving stories between Epics in JIRA does not sync to Portfolio

To reproduce:

• In JIRA:
  • Create epic with 2 stories
• In Portfolio:
  • Set "Keep backlog items in sync" for all issue types
  • Import into Portfolio
• Back in JIRA:
  • Create a new epic in JIRA, and move one of the stories to it

Expected behaviour:

• Portfolio should each Epic as having one story

Actual behaviour:

• First epic has both stories, even if you update the plan from JIRA

This is really annoying, as I can't keep my epics in sync and have little faith I'm looking at the right information for any epic in either tool.