The best way I have come up with is downloading the underlying data of maps manually off Portal, uploading them to the new Portal, then loading the Maps from the old Portal in Pro, switching the layer's data sources to the new Portal and then publishing the map to the new portal. Does anyone have a better solution?
As for why manually and not with Python, the clone_items method doesn't actually work and just creates useless references to the original item.
There isn't a one-size-fits-all solution. However, if you are running ArcGIS Enterprise 10.8.1, you can leverage this: