How to prevent Portfolio from re-ordering all JIRA stories within an epic?

[Schuyler Stockman]

In our current Agile Development Process, our Product Owner utilizes Portfolio for high-level backlog management of epics and 'initiatives' (level higher than epics). These epics (and the individual stories within them) are prioritized every sprint during a sit-down meeting with our PO, stakeholders, and higher level management in our company. Our team then takes the fully-prioritized product backlog in JIRA and defines what work we will take on for the next sprint in our Sprint Planning session. 

Our PO has noticed that sometimes when he changes the priority order of an epic, the priority of the stories within the epic will be shuffled randomly. To counteract losing the priority order of the stories we've been utilizing a custom JIRA issue field titled 'Sprint Order' and manually numbering each story's priority order, however this process is inefficient and can cause problems for team members reporting during our daily Stand Ups. 

I've read through a similar question on the Atlassian Community website and they raised a similar concern about Portfolio/JIRA ranking, but I was wondering if anyone had any suggestions on how we should deal with this problem?

Thank You!