The legend element in an ArcGIS Pro project does not show the newly added layer to an existing map using ArcPy.

Workaround

From our testing we found two workarounds:

1. This only happens for scripts run outside of Pro. Scripts in the Python Window work as well as script tools.
2. To remedy the problem outside of Pro, all you need to do is reference the layout first using

lyt = p.listLayouts()[0]   

BEFORE adding the layers to a map.

Steps to Reproduce