live_view_portal the npm package contains the client side code for LiveView with some modifications to make it work in third-party pages.
When you install phoenix_live_view from hex.pm you get both the server and the client side code. Each live_view_portal version is fixed to a phoenix_live_view version, becuase the server and client code should be able to “talk” to each other.
So to answer your question, it is not related to iframe. It is an inherent problem of the library. When we get this merged to LiveView this problem will of course be gone 
(I’m trying to find the time to make this happen).
