I have the same issue, I'll attempt to explain in detail. I configured my pop-up in ArcGIS Pro and used Arcade Expressions in popup Image url section to generate dynamic images. So something like this: https://exampleUrl.com/{expression/AnExample}. I then upload the Layer on Enterprise, as a reference map image layer and include a feature layer. When I attempt to view it through the Portal webmap viewer, the Map Image layer does not contain the expression anymore in the pop up. It becomes only https://exampleUrl.com/. However, the feature Layer contains the Arcade Expression and works fine, shows the Pop Up with the image. I am curious as to why the Map Image layer does not do that.
