Thank you for your post.
I am able to replicate the issue. When sliding to the slide panel that contains a portal with a vertical scroll bar enabled, the portal flashes in the current slide panel before the animation takes you to the portal slide panel.
I have sent your post along with my findings to our Development and Testing departments for review. When I receive any feedback, I will let you know.
An entry in the Known Bugs List has been linked to this Issue Report. Any Comments/Questions/Suggested Corrections should be posted here or in a new thread. Please do not post such comments to the Known Bugs List thread.