Nested Portfolio Instant Apps

[EricKlein1]

Our organization has been using nested Story Map Series apps to create multi-level menus for packages of maps, apps, and external web pages.  (For example, Story Map Series with tabs embedded within a Story Map Series with side accordion navigation).  When I try to add a section by pointing to another Portfolio Instant App, I get the following error.  Tried both selecting the target Portfolio using the ArcGIS option and by specifying its URL.  Same result:

Are nested Portfolio Instant Apps not supported?  If that's the case, what might be the best alternative to move forward now that Story Map Series is in mature support?

Thank you!