Scrollable frame overlays don't appear in the viewer when nested inside an MSO (Multi-State Object), also known as a Slideshow overlay type. This issue is unique to content authored with the v18 Folio Producer Tools and v18 viewers.
Remove the angle brackets from the frame's name in the layers panel. Do the following for each frame affected:
Adobe hopes to address this issue in a future release.
You can find a more detailed explanation of this problem and solution in this post on James Lockman's blog.