The result of the Add Join tool is not available in a map of the ArcGIS Pro project after running this method outside of ArcGIS Pro.

Additional Information

Workaround

There are three workarounds for this defect:

1) Using the Validate Join tool ( https://pro.arcgis.com/en/pro-app/latest/tool-reference/data-management/validate-join.htm) before the line of the Add Join tool and running the Python script. 

arcpy.management.ValidateJoin(in_layer_or_view=first_layer, in_field=" ", join_table=second_layer, join_field=" ")

Thus, the whole Python script is as follows:

# import ArcPy library

import arcpy

# access ArcGIS Pro project and the first map of the project

aprx = arcpy.mp.ArcGISProject('path to ArcGIS Pro Project')

my_map = aprx.listMaps()[0]

# access the two newly created feature classes 

first_lyr = my_map.listLayers()[0]

second_lyr = my_map.listLayers()[1]

# validate join

arcpy.management.ValidateJoin(in_layer_or_view=first_layer, in_field=" ", join_table=second_layer, join_field=" ")

# use AddJoin() method to join the two feature classes 

add_join_result = arcpy.management.AddJoin(in_layer_or_view=first_lyr, in_field=" ", join_table=second_lyr, join_field=" ", join_type="KEEP_ALL")

print(add_join_result)

# save ArcGIS Pro project

aprx.save()

2) Using Python environments of ArcGIS Pro 3.3 or ArcGIS Server 11.3.

3) Using the CopyFeatures() tool to save the result of join in a new feature class.

Steps to Reproduce